7.62x39 AR-15 bolt? - Page 4

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[ARCHIVED THREAD] - 7.62x39 AR-15 bolt? (Page 4 of 4)

Posted: 5/18/2007 11:50:17 PM EDT

[#1]

Quoted:
Here are the bolts from my Colt 7.62 upper and my CSS Grendel. The Colt was bought new in 1998 and the CSS barrel was received in December with what was advertised as a CMT bolt on the website. The Colt bolt is on the right.

i144.photobucket.com/albums/r172/ofreen/bolts2.jpg

Inside diameter on the Colt bolt is right at .450". The sidewall thickness as close as I could measure it is .053".

The CMT bolt inside diameter measures .448". Sidewall thickness is .045".

The bolts differ in that there is a slight flat milled between the lugs on the CMT bolt, where the Colt is round in profile. That seems to account for most of the difference in sidewall thickness.

Both bolt face recess depths are right at .125." Bolt lug length on both is .280."

??????? No difference in the Grendel and Colt bolt face depth? Is CSS making their own Grendel conversions with different specs than AA?

Posted: 5/19/2007 1:10:59 AM EDT

[#2]

Quoted:

??????? No difference in the Grendel and Colt bolt face depth? Is CSS making their own Grendel conversions with different specs than AA?

Yes. CSS uses regular 7.62 bolts.

Posted: 5/19/2007 4:44:46 AM EDT

[#3]

Quoted:
Ofreenbean or Shotgun1,
Can one of you be kind enough to tell me how your Colt bolts are marked. I have a bolt that was sold to me as a Colt, but it lacks the markings I'm used to seeing on my 5.56 bolts; the only marking on it is 762.

Brazos_Jack -- I was planning to list this bolt on the EE. IM me if you're interested.

I'll double check later but if I recall that's the only marking on mine. 7.62 lightly etched.

ETA; I checked. No other markings.

Posted: 5/19/2007 8:55:41 AM EDT

[#4]

Quoted:
Silly question I suppose, but do you know who makes the AA bolt - If you don't an educated guess would do? Apart from the deeper recess and the groove around the face of the DPMS bolt they look very similar - even the breech wall thickness is the same. Is there any difference in the metallurgy that's apparent?

I was told that it is Advanced Technology (name may be wrong, but something like that) in Florida.

Posted: 5/19/2007 9:10:36 AM EDT

[#5]

Never mind.
It's an optical illusion why the colt looks thicker.
The actual measurement only being 0.006" thicker is no major advantage.

It's no illusion and it's not the photo. The DPMS is thin in hand. .006 may not seem much but when you're dealing with an overall measurement of only .445 I think it is considerable. Just as the difference between the 7.62 and .556 wall thickness is small but considerable.

Posted: 5/19/2007 9:13:44 AM EDT

[#6]

Quoted:

Quoted:
Ofreenbean or Shotgun1,
Can one of you be kind enough to tell me how your Colt bolts are marked. I have a bolt that was sold to me as a Colt, but it lacks the markings I'm used to seeing on my 5.56 bolts; the only marking on it is 762

Brazos_Jack -- I was planning to list this bolt on the EE. IM me if you're interested.

I'll double check later but if I recall that's the only marking on mine. 7.62 lightly etched.

Roger that. Mine is lightly etched also. Thanks!

Posted: 5/19/2007 10:10:22 AM EDT

[#7]

Well another question. Is the 6.8 SPC bolt actually cut to fit the case or is it just a 7.62x39 bolt? If it is indeed cut to fit the slightly smaller case, would rebating the 6.5 Grendel case by that very little difference, give any significant strength to it over using a 7.62x39 bolt, or even an AA bolt, in the 6.5 Grendel?

Posted: 5/19/2007 11:03:51 AM EDT

[#8]

There's about 25 thousandths difference in the case head diameters.

The 6.8SPC is about .421" and the 7.62x39 is .447", give or take a little for clearances.

I haven't examined the cases for rebating, but assuming that you'd have enough rim left to get an extractor hooked onto, a dedicated handloader could probably rebate the 7.62x39 rims to fit a 6.8SPC bolt.
It doesn't actually change the bolt thrust any, but would beef up the bolt under the lugs, which might help some.

I don't really think that is a solution for the general public, but some handloaders might find it useful.

Rebating case heads can be tricky, because it can weaken the unsupported base of the cartridge case(depending on the thickness of the base area, and if you have to deepen the groove for the extractor), and sometimes case blow-outs can occur at the rebated areas if you have enough pressure. This is a procedure for those who understand cartridge design, and have advanced experience in this field.

Posted: 5/20/2007 12:13:31 PM EDT

[#9]

Quoted:
There's about 25 thousandths difference in the case head diameters.

The 6.8SPC is about .421" and the 7.62x39 is .447", give or take a little for clearances.

I haven't examined the cases for rebating, but assuming that you'd have enough rim left to get an extractor hooked onto, a dedicated handloader could probably rebate the 7.62x39 rims to fit a 6.8SPC bolt.
It doesn't actually change the bolt thrust any, but would beef up the bolt under the lugs, which might help some.

I don't really think that is a solution for the general public, but some handloaders might find it useful.

Rebating case heads can be tricky, because it can weaken the unsupported base of the cartridge case(depending on the thickness of the base area, and if you have to deepen the groove for the extractor), and sometimes case blow-outs can occur at the rebated areas if you have enough pressure. This is a procedure for those who understand cartridge design, and have advanced experience in this field.

I believe if I did that I'd use the actual Grendel case because it is small rifle primed, not large rifle like most all 7.62x39 brass. This would give more support. We're not talking much of a rebate here. Any reduction in case head diameter has to equate into some less bolt thrust. If not having a heavier bolt, especially in the lug support area is comforting.

Posted: 5/20/2007 1:06:46 PM EDT

[#10]

Quoted:

I believe if I did that I'd use the actual Grendel case because it is small rifle primed, not large rifle like most all 7.62x39 brass. This would give more support. We're not talking much of a rebate here. Any reduction in case head diameter has to equate into some less bolt thrust. If not having a heavier bolt, especially in the lug support area is comforting.

The computation for thrust is related to the area of the INTERIOR portion of the base of the case(where the pressurized gas acts against the case), times the pressure.
You could rebate the case rim down to a pinhead, and the thrust against the rear of the interior of the case will be the same. It would just be distributed over a smaller area of the reduced-size rim/base, causing higher psi loading of the area that contacts the bolt face. So, in a way, rebating actually increases the load on the bolt.

What you gain from this is more meat in the bolt, supporting the lugs, so hopefully they would be less likely to break off under the load.

No free lunch.

Posted: 5/21/2007 10:15:10 AM EDT

[#11]

I know this is not strictly a bolt question, but are the barrel extensions different for the 7.62x39? I am thinking that the feed ramps in particular are likely to be different. Is there any chance someone could post some pictures so that we can compare the differences side by side between the 5.56 and 7.62x39 extensions. I know JFA (aka Old Joe on the Grendel site) had some problems with his barrel extension on his Grendel, and I am guessing the extension for that might be different also.

Posted: 5/21/2007 11:01:18 AM EDT

[#12]

Quoted:
I know this is not strictly a bolt question, but are the barrel extensions different for the 7.62x39? I am thinking that the feed ramps in particular are likely to be different. Is there any chance someone could post some pictures so that we can compare the differences side by side between the 5.56 and 7.62x39 extensions. I know JFA (aka Old Joe on the Grendel site) had some problems with his barrel extension on his Grendel, and I am guessing the extension for that might be different also.

I don't know about the 7.62x39, but the 6.5 Grendel certainly has different feed ramps and you are correct that I did have trouble. My CSS feed ramps were for the 5.56. I had to open them up to widen them a little for the 6.5 Grendel. That required removing the barrel. They feed wonderful now, everything including my blunt nosed cast loads. I would almost have to guarantee the 7.62x39 has a slightly altered feed ramp.

It's not that the extensions are different, the feed ramps are contoured differently.

Posted: 5/21/2007 11:13:45 AM EDT

[#13]

Can you post a picture of your reworked extension - if it was a 5.56 you must have opened it up some?

Maybe Randall, if he is about, could post pictures of the 5.56, 7.62x39 and Grendel extensions side by side.

Posted: 5/21/2007 11:54:08 AM EDT

[#14]

Quoted:
Can you post a picture of your reworked extension - if it was a 5.56 you must have opened it up some?

Maybe Randall, if he is about, could post pictures of the 5.56, 7.62x39 and Grendel extensions side by side.

I never took a picture after the rework, but here is the before one

Posted: 5/21/2007 12:28:13 PM EDT

[#15]

It's hard to tell without seeing a comparison - before and after, or alongside a 5.56 extension. If you have a camera to hand can you take a picture of your reworked version and post it?

Posted: 5/21/2007 1:26:38 PM EDT

[#16]

Posted: 5/21/2007 2:08:58 PM EDT

[#17]

Is there any difference then between a 5.56 and 7.62x39 extension? On the Grendel forum they seem to think there is with the 6.5. Is there anyway you can get a picture of the 5.56, 7.62x39 and 6.5 Grendel extensions side by side for comparisons?

I am intrigued to know why the M4 extension is so different, surely reliability would be improved if all uppers were built with extended feed ramps.

Posted: 5/21/2007 2:46:54 PM EDT

[#18]

Quoted:
Is there any difference then between a 5.56 and 7.62x39 extension? On the Grendel forum they seem to think there is with the 6.5. Is there anyway you can get a picture of the 5.56, 7.62x39 and 6.5 Grendel extensions side by side for comparisons?

To the best of my knowledge, there's no such thing as a 7.62x39 barrel extension.
All of the ones I have seen were the same as regular 5.56 extensions.

The AA 6.5 Grendel upper I have does not have extended feedramps either.
I have to look closer at the feedramps though to see what kind of radius they are.

I suspect that they are a 5/32" or 3/16" radius while regular extensions use a 1/8" radius.

I'm sure this has more to do with the shoulder of the case vs. the bullet.

6.8's run perfect with M4 feedramps.

Posted: 5/21/2007 2:55:18 PM EDT

[#19]

Quoted:

Quoted:
Is there any difference then between a 5.56 and 7.62x39 extension? On the Grendel forum they seem to think there is with the 6.5. Is there anyway you can get a picture of the 5.56, 7.62x39 and 6.5 Grendel extensions side by side for comparisons?

To the best of my knowledge, there's no such thing as a 7.62x39 barrel extension.
All of the ones I have seen were the same as regular 5.56 extensions.

The AA 6.5 Grendel upper I have does not have extended feedramps either.
I have to look closer at the feedramps though to see what kind of radius they are.

I suspect that they are a 5/32" or 3/16" radius while regular extensions use a 1/8" radius.

I'm sure this has more to do with the shoulder of the case vs. the bullet.

6.8's run perfect with M4 feedramps.

Randall,

I don't think the wider radius of the 6.5 feed ramp has to do to with with as much the shoulder of the cartridge, but more to do with the nose of the bullet to hit it in order to guide the cartridge. That feed ramp radius is no ways near wide enough to correspond with the 6.5's much wider radius. My problem with my CSS 5.56 feed ramps was the tips of the bullets hitting alonside the ramp and effectively jaming the rifle. After opening them up some the noase of the bullet hit inside the ramp and the cartridges fed good.

Posted: 5/21/2007 3:19:32 PM EDT

[#20]

Quoted:
I don't think the wider radius of the 6.5 feed ramp has to do to with with as much the shoulder of the cartridge, but more to do with the nose of the bullet to hit it in order to guide the cartridge. That feed ramp radius is no ways near wide enough to correspond with the 6.5's much wider radius. My problem with my CSS 5.56 feed ramps was the tips of the bullets hitting alonside the ramp and effectively jaming the rifle. After opening them up some the noase of the bullet hit inside the ramp and the cartridges fed good.

I don't think the nose WIDTH matters much.
M4 ramps work better because they sit LOWER in the receiver.
With larger diameter cases, this is particularly important because the tip of the bullet sits LOWER in the magazine.
Almost any bullet except a 6.5mm wadcutter would slide up a regular feedramp.
Remember, only the bullet TIP touches the feedramp.
The bullet does not slide parallel to the ramp.
The neck and body hit the ramp next while the cartridge is released from the magazine to go around the corner into the chamber.

By widening the ramp, the case does not go into the chamber with as steep of an angle.

Posted: 5/21/2007 3:27:17 PM EDT

[#21]

Quoted:

Quoted:
I don't think the wider radius of the 6.5 feed ramp has to do to with with as much the shoulder of the cartridge, but more to do with the nose of the bullet to hit it in order to guide the cartridge. That feed ramp radius is no ways near wide enough to correspond with the 6.5's much wider radius. My problem with my CSS 5.56 feed ramps was the tips of the bullets hitting alonside the ramp and effectively jaming the rifle. After opening them up some the noase of the bullet hit inside the ramp and the cartridges fed good.

I don't think the nose WIDTH matters much.
M4 ramps work better because they sit LOWER in the receiver.
With larger diameter cases, this is particularly important because the tip of the bullet sits LOWER in the magazine.
Almost any bullet except a 6.5mm wadcutter would slide up a regular feedramp.
Remember, only the bullet TIP touches the feedramp.
The bullet does not slide parallel to the ramp.
The neck and body hit the ramp next while the cartridge is released from the magazine to go around the corner into the chamber.

By widening the ramp, the case does not go into the chamber with as steep of an angle.

With either one or two rounds in a Grendel magazine looking at the mag from the front, the bullet tip point should be 1/8th inch above that front mag wall and mine are dead on. In that respect, that tip doesn't hit the feed ramp low, in fact, other then being wider, the M4 feed ramp is really needed.

Posted: 5/21/2007 4:47:41 PM EDT

[#22]

So 6.8s use M4 extensions to cater for the wider bullet then? I understood you couldn't use an M4 extension in a standard upper because of the extended feed ramp. Looking at the Grendel forum it looks like this is what's being used for the Grendel:

http://i81.photobucket.com/albums/j205/areeves08/IMG_0754.jpg?t=1173461039

The extension in this picture looks like the M4 version Randall posted. The rifle in question was a T16, which is an Alexander Arms gun, I think.

Posted: 5/21/2007 6:59:14 PM EDT

[#23]

Quoted:
So 6.8s use M4 extensions to cater for the wider bullet then? I understood you couldn't use an M4 extension in a standard upper because of the extended feed ramp. Looking at the Grendel forum it looks like this is what's being used for the Grendel:

http://i81.photobucket.com/albums/j205/areeves08/IMG_0754.jpg?t=1173461039

The extension in this picture looks like the M4 version Randall posted. The rifle in question was a T16, which is an Alexander Arms gun, I think.

If you use an extension with M4 feed ramps, the lip of the ramp doesn't come to the edge of the regular upper receiver. In you photo (I had to lighten it up to see) they appear that they do. That's what is wrong with using an M4 upper with a normal ramped extension, the bullet tip sometimes catches that edge because the receiver is grooved out.

Posted: 5/21/2007 11:42:01 PM EDT

[#24]

I had a great post all composed before Arfcom went down tonight and I even took some new pictures, but I lost the text of the post.

So at least here is my new feedramp picture...

Posted: 5/22/2007 4:27:43 AM EDT

[#25]

There you go, a picuture is worth a thousand words.

Posted: 5/22/2007 8:45:26 AM EDT

[#26]

That explains everything, thanks Randall. JFA said that he had to rework the ramp angle on his Grendel's extension, something about the wider case body and angle the bullet runs up the feed ramp being too sharp. It would be interesting to see what this looks like.

Posted: 5/23/2007 9:59:19 PM EDT

[#27]

...and now back to our regularly scheduled program.

I finally have a Colt 7.62 bolt inbound, but I still need a second.

Anyone who knows of one for sale, please IM or email me.

Posted: 5/24/2007 6:23:54 AM EDT

[#28]

Quoted:
I finally have a Colt 7.62 bolt inbound,

I really think thats the best bolt you can get at the present time. Maybe when & if MGI comes out with their Hi-strength bolt we will truly have a better bolt. M9

Posted: 5/24/2007 9:25:40 AM EDT

[#29]

Brazo where did you find your Colt bolt?

Posted: 5/24/2007 10:36:00 AM EDT

[#30]

Quoted:
Brazo where did you find your Colt bolt?

One of the guys here IM'd me. It was out of a parted out Colt (or maybe a caliber change project).

I'm hoping someone else with a Colt 7.62x39 bolt will see this and IM me so I can get the second 7.62 bolt I need.

Posted: 6/1/2007 9:44:13 AM EDT

[#31]

bump

Posted: 8/8/2007 4:57:24 AM EDT

[#32]

BTT

I'm still trying to find someone selling a Colt 7.62x39 bolt.

Anybody here got one to sell?

Posted: 8/8/2007 5:11:29 AM EDT

[#33]

Quoted:
BTT

I'm still trying to find someone selling a Colt 7.62x39 bolt.

Anybody here got one to sell?

Try calling Ken Elmore at SAW Sales.
He's a big Colt dealer, and he may have some left.

Posted: 8/8/2007 1:36:10 PM EDT

[#34]

Does anyone have a source for CMT bolts in 7.62x39?

Posted: 8/13/2007 1:17:34 PM EDT

[#35]

Quoted:
...and now back to our regularly scheduled program.

I finally have a Colt 7.62 bolt inbound, but I still need a second.

Anyone who knows of one for sale, please IM or email me.

Sheesh - 8 pages of hoo ha over a bolt.

Why not just buy one from Olympic Arms? I have one made by them from a few years back when I intended to build a 7.62x39mm but never got around to it (then ammo prices went over $100 a case delivered - outrage!).

Mines not for sale, but couldn't Oly just sell you one?

Posted: 8/13/2007 5:48:17 PM EDT

[#36]

Quoted:

Mines not for sale, but couldn't Oly just sell you one?

Midway sells OLY bolts as a in stock ready to ship item. Getting an OLY bolt is not a problem. The problem is bolts with this bolt face are weak. The Colt bolt is generally considered quite a bit superior. They are however discontinued. M9

Posted: 8/13/2007 8:23:29 PM EDT

[#37]

Quoted:

Quoted:

Mines not for sale, but couldn't Oly just sell you one?

Midway sells OLY bolts as a in stock ready to ship item. Getting an OLY bolt is not a problem. The problem is bolts with this bolt face are weak. The Colt bolt is generally considered quite a bit superior. They are however discontinued. M9

Exactly,

The stresses are considerably higher on 7.62 bolts compared to 5.56. If someone else would make their bolts like Colt - shot peened, proof tested, and magnafluxed - I would by them. That ain't Oly.

Posted: 8/13/2007 8:27:10 PM EDT

[#38]

ok thanks for the explanation. Well I am still going to use mine anyway -even if every member of arfcom thinks Oly sucks. I don't care. We will see if they are OK or not.

Posted: 8/13/2007 9:08:13 PM EDT

[#39]

Quoted:
ok thanks for the explanation. Well I am still going to use mine anyway -even if every member of arfcom thinks Oly sucks. I don't care. We will see if they are OK or not.

keep us posted how it works...

Posted: 8/13/2007 9:12:52 PM EDT

[#40]

height=8

Quoted:
height=8

Quoted:

Mines not for sale, but couldn't Oly just sell you one?

Midway sells OLY bolts as a in stock ready to ship item. Getting an OLY bolt is not a problem. The problem is bolts with this bolt face are weak. The Colt bolt is generally considered quite a bit superior. They are however discontinued. M9

ArmaLite's solution to their AR10 bolts might be the solution. I wonder if 7.62x39 bolts could be modified so that the stresses could be equally shared across all the bolt lugs.

http://www.armalite.com/library/techNotes/tnote13.htm

http://patft.uspto.gov/netacgi/nph-...8&RS=PN/6044748

Posted: 8/14/2007 10:14:06 AM EDT

[#41]

1. ArmaLite® bolts bear 7 locking lugs, each 22.5 degrees apart from the next. At this angle there should be 8 lugs in a circle, but one lug has been removed to provide space for the extractor.

2. The missing lug creates an asymmetry, or imbalance, in the bolt. When the rifle is fired the pressure of the cartridge presses the bolt rearward, and the lugs transfer this energy to the matching locking lugs of the barrel extension. The bolt flexes at the rear inside corner of the lug during this process. Because one lug is missing, the deformation is greatest in the area of the missing bolt. A series of engineering analyses have disclosed that the lugs on either side of the extractor each receive 40% of the recoil load.

3. Relieving the rear of the lug opposite the extractor "balances" the loading of the bolt by restoring symmetry. The load placed on the bolt is spread more evenly over the six remaining lugs. Peak lug loading is reduced.

4. Relieving the lug opposite the extractor reduces peak loading on the lugs neighboring the extractor from 40% to 24% each. It is not reduced to 16 1/3% (1/6 of the recoil load) because a small asymmetry remains: the lugs neighboring the extractor are slightly undercut for the extractor, which results in continued relative weakness at those lugs. Nonetheless, the load on the weakest lugs is reduced 40%, compared to the standard M-16 design, by spreading a portion of that load to the other 4 lugs.

5. In addition to the relieved lug, the AR-10B™ bolt bears tapered lugs with wide roots. This puts added strength exactly where stresses are at their greatest. The AR-10® bolt is significantly stronger than the similar M-16 bolt. A patent covering both bolt and barrel extension relief is pending.

I think you are onto something, also could the AR10 bolt have a carrier modified to fit into the AR15 receiver?????( I assume the carriers are different)

If that would work we could see a .260rem/.308win/.243win/7mm-08 AR15 really soon.

I bet that Armalite has quite a few bolts on hand and price can be negotiated

Paging TWL!!!!

Posted: 8/14/2007 10:42:03 AM EDT

[#42]

Bolts and barrel extensions work together.
AR10 barrel extensions don't fit in AR15 receivers.
Some mods are possible by doing machine work to the receiver and barrel extension. Oly is using something like this in some of their models.

We're not going in that direction at the present time.

Posted: 8/14/2007 10:58:04 AM EDT

[#43]

NP was just a thought.

Posted: 8/14/2007 5:48:47 PM EDT

[#44]

i have dpms 7.62 x 39 rifle. never had any trouble. they sale the bolt stripped or assembled. assembled is only like $69. i have always had good luck out of dpms panther parts. thats pretty much all i buy any more.

Posted: 8/15/2007 10:11:04 AM EDT

[#45]

I will admin I didn't read all 8 pages so this might have already been covered in some form. Is there any reason why the LW bolt isn't as good as the COLT bolt? BTW, I have both and run both in full auto and yet to have a problem with either.

Posted: 8/15/2007 11:39:50 AM EDT

[#46]

Quoted:
I will admin I didn't read all 8 pages so this might have already been covered in some form. Is there any reason why the LW bolt isn't as good as the COLT bolt? BTW, I have both and run both in full auto and yet to have a problem with either.

No reason except unavailability.

Posted: 8/15/2007 12:32:31 PM EDT

[#47]

Yes,

I think the LW ACB is excellent, but has been out of production for some time in both calibers. I contacted them and they are planning to get the 5.56 back in production as soon as they can get the "unobtainium" they make the bolts from. Getting the 7.62 ACB back in production isn't even on their radar right now.

[ARCHIVED THREAD] - 7.62x39 AR-15 bolt? (Page 4 of 4)

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