Gas port size in early Bushmaster 10.5 inch (in Kanada this is okay)

AR-15 » Rifles, Uppers, Lowers, Barrels, and more

AR Sponsor:

Site Notices

[ARCHIVED THREAD] - Gas port size in early Bushmaster 10.5 inch (in Kanada this is okay)

Posted: 4/2/2005 6:31:57 PM EDT

I went to the range yesterday with my shorty and it turned out to be a single shot. I had installed a mag-pul buffer and spring suitable for a M9 buffer tube. Now when I got the barrel short butt combo, many years ago, it came with a lighter buffer spring and a lighter buffer when compared to the mag-pul. I measured my gas port in the barrel and it is 0.0625" (1/16"). How does this port size compare to say a modern shorty or an M4 configuration. My front sight retaining pins are straight as well. Info would be appreciated.

Posted: 4/3/2005 5:44:05 AM EDT

[#1]

Quoted:
How does this port size compare to say a modern shorty or an M4 configuration.

The closest info I could find is for an 11.5" barrel. If the barrel diameter is .625", then the gas port should be between .081" and .089". If the barrel diameter is .750", then the gas port should be between .086" and .094".

Posted: 4/4/2005 8:27:53 AM EDT

[#2]

Thanks for the info, next time out I will use the buffer and spring I got with the barrel. If it isn't broken....don't fix it. I will continue to research gas port hole sizes. Good info in 'Black Rifle' vol 1 & 2, getting things to work right is 70% of the fun anyhow as I have to drive 105 miles to do a test fire with my five round magazine.
Be safe, shoot straight
WetCoastBob

Posted: 4/4/2005 1:28:44 PM EDT

[#3]

The situation is that you are probably just barely getting enough gas with the light buffer. Adding the heavier buffer was just too much weight to cycle.

Posted: 4/4/2005 1:31:41 PM EDT

[#4]

Quoted:
I went to the range yesterday with my shorty and it turned out to be a single shot. I had installed a mag-pul buffer and spring suitable for a M9 buffer tube. Now when I got the barrel short butt combo, many years ago, it came with a lighter buffer spring and a lighter buffer when compared to the mag-pul. I measured my gas port in the barrel and it is 0.0625" (1/16"). How does this port size compare to say a modern shorty or an M4 configuration. My front sight retaining pins are straight as well. Info would be appreciated.

I'd open it too .090" and see how it runs. I did that to a 10.25" barrel that would run fine with a suppressor but wouldn't without it. It's gas port was .080", I believe it was a cutdown barrel. Runs great now!

Posted: 4/4/2005 6:29:45 PM EDT

[#5]

Sounds good to me, I will try the lighter buffer and spring first and then start opening up the gas port as required, as small an increment as I can. I once had excellent success with a 5.56 AK by making small 'jets' to screw in to the huge gas port (10-32 thread) that I tapped into it. Then I bought a real gun. I prefer the AR, sort of a small block Chevy you can 'play' with. Due to distance to range, I will bring all the spares and tools the next time I try it! Thanks guys.

Posted: 4/5/2005 5:54:48 AM EDT

[#6]

CRANE CQB-R 10.3" UPPERS RUN A 0.070" GAS PORT.

LMT USES A 0.073" GAS PORT IN THEIR 10.5" UPPERS.

BOTH ARE UTILIZED WITH "H" BUFFERS.

WES

Posted: 4/5/2005 10:58:23 PM EDT

[#7]

Hey big thanks Wes, you concur with a local Gunsmith .'Dlask Arms' gave me similar information saying go easy for increases over 0.070". I was very pleased with the fit and function of the lower receiver I got from Dlask. Thanks for the info on the shorty's, I look forward to the next function test. From what I've been told so far, from you guys that know,I'll be able to get that shorty running sweet.
wetcoastbob

Posted: 5/4/2005 4:25:12 PM EDT

[#8]

Second trip to range today, gonna have to open up that gas port as the shorty (10.5) was not getting enough gas to drive the bolt carrier aft. This was incorporating the spring and buffer I got when I purchased the shorty kit. This spring is weaker than a standard and the buffer is synthetic with a nylon bumber and some lead weight inside.I even took the weight out of the buffer and it still short cycled. The spent case extrated and ejected perfectly. Now I am going to open up the gas port to 0.070" and see if the increase helps me with my cycling. The 20 inch barrel with standard M9 tube and spring worked flawlessly. If any of you guys that really know this stuff have some more pointers.....lemme have em.
thanks

Posted: 5/9/2005 2:49:45 PM EDT

[#9]

Another fun filled day at the range. Close but no cigar. With 0.070 gas port size and an 80 gram shorty M93 (magpul) buffer,magpul spring and an M9 buffer tube, the gun fired and ejected the spent case but failed to pick up the next round. I think the bolt and carrier may be travelling forward perhaps too quickly to pick up another round? When the 20' standard upper was installed, the same magazine performed flawlessly. From other topics posted, I see that the 9mm carbine bufferr seems to be the ticket. I dont want to open the gas port any larger. Any suggestions all you RKI's? As far as I'm concerned, making things run right is almost as much fun shootin! At least in Kanada with our five round magazines.

Posted: 5/9/2005 4:39:16 PM EDT

[#10]

Quoted:
Another fun filled day at the range. Close but no cigar. With 0.070 gas port size and an 80 gram shorty M93 (magpul) buffer,magpul spring and an M9 buffer tube, the gun fired and ejected the spent case but failed to pick up the next round. I think the bolt and carrier may be travelling forward perhaps too quickly to pick up another round? When the 20' standard upper was installed, the same magazine performed flawlessly. From other topics posted, I see that the 9mm carbine bufferr seems to be the ticket. I dont want to open the gas port any larger. Any suggestions all you RKI's? As far as I'm concerned, making things run right is almost as much fun shootin! At least in Kanada with our five round magazines.

Sounds like it's short stroking (moving far enough for ejection but nor far enough to scoop up the next round). To see if it is, just load 1 round and see if it'll lock the bolt back when empty. If it does it's moving far enough rearward.

If it is short stroking I'd try a regular CAR buffer, them if it seems like it runs too fast and/or gives you extraction trouble use an H buffer.

Posted: 5/9/2005 6:04:30 PM EDT

[#11]

80gm is only like 2.8oz. Not picking up the next round sounds like short-stroking and with such a light buffer I'd be looking at the gas system. Putting in an H buffer or something heavier is going to cause more problems.

www.biggerhammer.net/ar15/buffers/

Do you have a good gas seal at the front sight gas port where the gas tube goes in? Is there much residue blowing past?

Good bolt gas ring seal?

You want to solve more rearward travel first rather than worry about slowing things down front or rear.

Posted: 5/9/2005 7:12:58 PM EDT

[#12]

Quoted:
80gm is only like 2.8oz. Not picking up the next round sounds like short-stroking and with such a light buffer I'd be looking at the gas system. Putting in an H buffer or something heavier is going to cause more problems.

www.biggerhammer.net/ar15/buffers/

Do you have a good gas seal at the front sight gas port where the gas tube goes in? Is there much residue blowing past?

Good bolt gas ring seal?

You want to solve more rearward travel first rather than worry about slowing things down front or rear.

I think you're right I misread his post I thought he was using a 9mm really heavy buffer.

It sounds like he's using a regular CAR buffer already and having short stroke problems. I think the port needs just a little opening maybe too (.080") if that don't work (.085") and if that don't work (0.90") like I had to go on a 10.25" barrel that I had to.

Posted: 5/11/2005 2:54:04 PM EDT

[#13]

I do not think that it is short stroking. The first round fired and ejected properly, the carrier is moving fully rearward, the hammer is cocked and i get a hammer fall on the second trigger pull with no round in chamber. Before I opened up the gas port to 070, it was definately doing the short stroke. I was amazed at the difference of opening it up a mere 0.008". The carrier and bolt move rearward with much more ferocity for want of a better word. It sounds like the BC is moving fully rearward. Before I open it up anymore, simplest,quickest, cheapest is to play with buffer weights as one of you guys pointed out, an 80 gram buffer is not very heavy to begin with. Keep those ideas coming as I know enough to get myself in deep doo doo. I'll be posting any new changes and test fires in this post. One day it might help out another member. Thanks guys!

Posted: 5/11/2005 7:59:52 PM EDT

[#14]

Quoted:
Another fun filled day at the range. Close but no cigar. With 0.070 gas port size and an 80 gram shorty M93 (magpul) buffer,magpul spring and an M9 buffer tube, the gun fired and ejected the spent case but failed to pick up the next round. I think the bolt and carrier may be travelling forward perhaps too quickly to pick up another round? When the 20' standard upper was installed, the same magazine performed flawlessly. From other topics posted, I see that the 9mm carbine bufferr seems to be the ticket. I dont want to open the gas port any larger. Any suggestions all you RKI's? As far as I'm concerned, making things run right is almost as much fun shootin! At least in Kanada with our five round magazines. hr

I had the same exact trouble your having after installing a new heavier carrier. It would fire and eject the spent case but the bolt/carrier would not travel back far enough to catch and strip a new round for the reload. I had to open up the port to .086 for it to cycle properly.

Posted: 5/21/2005 11:49:09 AM EDT

[#15]

20 May 05, went to the range again today, looks like I ate a big helpin of 'crow', the gas port at 0.070" is still too small. You fellows that know seem to think it was short strokin.....me thinks you are right! I'll try and take a rechargable drill to the range next time. I want to open it up to 0.075" and see what haps, if still no joy, I'll open up to 0.080". I just dont want to make it too big, they are no extraction problems what so ever so far. Thanks for all the good advice.
wetcoastbob

Posted: 5/21/2005 3:39:28 PM EDT

[#16]

Quoted:
20 May 05, went to the range again today, looks like I ate a big helpin of 'crow', the gas port at 0.070" is still too small. You fellows that know seem to think it was short strokin.....me thinks you are right! I'll try and take a rechargable drill to the range next time. I want to open it up to 0.075" and see what haps, if still no joy, I'll open up to 0.080". I just dont want to make it too big, they are no extraction problems what so ever so far. Thanks for all the good advice.
wetcoastbob

I think I'd go about .0025 to .0050 more than where it just works just to be safe incase you had to use weak ammo in an emergency. I opened a buddies 10.25" upper from 0.080 to 0.090, it now works with or without a suppressor on it, no extraction problems whatsoever. Before it would only work with a suppressor.

Posted: 5/23/2005 12:33:14 PM EDT

[#17]

Went to the range today, yipee, got my shorty runnin sweet. Opened up the gas port to 0.080" and used a standard car buffer and spring the little fellow ran great and my eotech let me put over 10 rounds into a 2.5 inch circle at 100 yards. I may open it up another 0.002" just to ensure function with non mil spec ammo. At least it works, maybe the thread can be closed now....thanks for all the help guys!
wetcoastbob

Posted: 5/23/2005 3:28:28 PM EDT

[#18]

Quoted:
Went to the range today, yipee, got my shorty runnin sweet. Opened up the gas port to 0.080" and used a standard car buffer and spring the little fellow ran great and my eotech let me put over 10 rounds into a 2.5 inch circle at 100 yards. I may open it up another 0.002" just to ensure function with non mil spec ammo. At least it works, maybe the thread can be closed now....thanks for all the help guys!
wetcoastbob

Good to read you gotter workin!

[ARCHIVED THREAD] - Gas port size in early Bushmaster 10.5 inch (in Kanada this is okay)

AR-15 » Rifles, Uppers, Lowers, Barrels, and more

AR Sponsor:

Warning

Confirm Action

About AR15.COM

Stay Connected

Newsletter

Contact Us