AR Sponsor
Posted: 12/27/2004 7:59:59 PM EDT
|
Hey guys, I'm having some trouble with the rifle I've put together. The only ammo my rifle seems to feed reliably is 62gr ammo. This can be either cheap winchester stuff, or green tip penetrators. With other ammo, the bolt never fully closes on its own. I checked the headspace, and everything looked fine. When cycling rounds through by hand ( just cycling the action, not firing ), I noticed that the bullets all have deep scratches on one side. Is that normal? Anyway, here are some specifics: DPMS A3 upper receiver Olympic Arms Stainless UltraMatch barrel ( SUM ) 20" 1:8" with minimum SAAMI spec chamber Les Baer bolt and bolt carrier DPMS Miculek Steel charging handle DPMS forged lower Any help would be much appreciated. Thanks. Sam |
That's your problem. Olympic is notorious for small chambers. If you want a general shooter that likes all types of ammo, replace it with a Colt or Bushmaster chrome lined barrel. Your pal, HE |
|
Well, I'm okay with it being particular. I built it as a target rifle. It bugs me that it only seems to like military surplus and cheap winchester white box ammo. I'd really prefer it to reliably feed the Gold Medal Match, Hornady Match, and Black Hills Match that I've been trying to shoot in it. Sam |
Then you need to get a reamer and cut the freebore a little longer. What most people do is adjust the bullet seat depth on their handloads so the bullet just barely touches the rifling. Loads that show engraving on the bullets are too long for this barrel. Basically you're asking this barrel to chamber loads it wasn't designed to. |
|
I have a set of Forster head space gauges. What's a 45 degree drop test? I don't feel any binding when I cycle the action without ammo. The bolt simply doesn't go fully into battery. It will stop somewhere around 1/8" to 1/4" out of battery with 69gr Gold Medal Match. There are scratches on the cases from the magazine, but the scratches that I mentioned are on the bullet itself. The scratch starts a little back from the tip of the bullet and continues back to nearly the case mouth. Sam |
Well, there's the thing. The scratch on the bullet really doesn't look like engraved rifling to me, it just looks like a nasty scratch. A scratch that only exists on one side of the bullet. I'm not sure what the chamber was designed for, but with a 1:8" twist, you'd think it would be designed for heavier/longer bullets. BTW, I can't feed chamber 55gr bullets at all. I seem to be able to chamber any/all 62gr bullets. I also can't chamber the 68gr and 69gr match bullets that I intended to shoot in this rifle. Also, these are not handloads. These are "off the shelf" match grade rounds. I tried to post a picture of the scratch, but screwed it up and managed to delete the first attempt at this message, so I think I'll leave that alone for now. Thanks again. Sam |
|
the feed ramps might be dinging up the mouth of the case and causing the case to stick in the chamber. you seeing damage like that during feeding? look for a burr on the feed ramps. look for scuffs on the upper below the feed ramps. if the projo gets set back into the case the bolt could hang during chambering. shotgun the rifle, hold the barrel so it's 45 deg to the ground, retract the bolt by hand and let it slide shut. the bolt should close under it's own weight. I guarantee that the throat is already long enough for GM or any other load. Far too long if you want the ogive just off the lands. GM tends to the small side fo the dims unlike USGI loads. an 1/8" to a 1/4" gap isn't a matter of cartridge and chamber interference. to clarify, the rifle will not live fire or bench test the GM rounds? |
I don't really see any scuffs on or near what I assume to be the feed ramps. Above that a see a couple of shiny spots that look like they might correspond to the locking lugs. The bolt will close under its own weight at near 45 degrees. I'm not a great judge of angles. It doesn't take much variation from my guess at 45 degrees for the bolt to stop just out of battery. When closing the bolt by hand, I don't notice any roughness or binding. When cycling the bolt manually, with GM 69gr, the bolt will never fully close. When live firing, the bolt will fully close less that 50% of the time. Sometimes approaching 25% or less. Bullet setback does not appear to be an issue. A couple of rounds out of a new box showed a nominal COAL of 2.250", and the ones that were cycled through show a nominal COAL of 2.249" The scratch on the bullet does not extend all the way to the case mouth, but just shy. I can see any marks on the case mouth itself. Thanks again. Sam |
The bolt/bolt carrier is the one piece that I ordered pre-assembled; so I'm not real sure how to tear it apart. I see a cotter pin that holds the firing pin in place, but nothing obviously holding the bolt in. Do I need to remove the piece that fits in the gas tube? It seems to be staked in place. The scratches are indeed present on the ammo that works. sam |
|
pull the firing pin retaining pin, remove the firing pin from the rear, rotate the cam pin 90 deg, push the bolt into the locked position, pull the cam pin out, remove the bolt from the carrier. after that download the -10 and read it. it'll save a lot of confusion. all I can think (if it's not a mechanical problem) is a neck size interference issue between the chamber and the round. that doesn't sound right tho bc the GM loads are smaller (at least IME) than the USGI, or surplus stuff. you said you had a set (GO NOGO FIELD) of Forster gages, how did you use them? walk me through it. |
First off, I don't have a field gauge, just the go and nogo gauges. Do I need the field gauge? I start with the barrel separate from the upper receiver. I put the go gauge behind the extractor in the bolt and push the gauge into the chamber. With the go gauge, the locking lugs are fully engaged and the bolt carrier touches the barrel extension. With the nogo gauge, the locking lugs are not fully engaged and the bolt body does not touch the barrel extension. Sam |
I do notice some resistance when the bolt carrier approaches the forward edge of the ejection port, about half an inch back. That's where those shiny spots in the inside of the upper are. That doesn't really explain the scratches on the bullets, but maybe that's a separate problem. Do you think the carrier rubbing on the inside of the upper is my problem? If so, what can I do about it? Will I have to get another upper? Thanks again. Sam |
|
Do you have another bolt/ carrier combo? Also, have you tried loading individual rounds with finger pressure, to determine where the scratches come from? i.e. if the scratches are consistently at the same clock position, maybe a bur in the chamber/ leade. HTH, Bryan |
|
Nope, the Les Baer carrier group is the only one I have. I tried dropping a couple of rounds in the chamber while I had the barrel removed. Everything I have seems to drop in fine. No binding, no scratches. I even tried hooking a round in the bolt while it was removed from the carrier and locking it in place. No binding, no scratches. I'm beginning to think that the bolt carrier rubbing against the upper receiver is reducing its velocity to the point where it doesn't have the energy to engage the locking lugs. I'm not sure what to do about that, though. I guess I could polish the inside of the receiver with Flitz where it's rubbing. Still no idea where the scratches are coming from. I don't see any copper left on the upper receiver. Any ideas? Thanks. Sam |
|
Hello all, Well, I took a look at the inside of the upper receiver where those shiny spots were. There apears to be a ledge of some sort right there. Is that normal? It's just in front of some sort of cutout in the upper receiver, maybe an inch back from the front of the receiver. I tried relieving that ledge a little, but the bolt carrier still drags when it comes in contact. Suggestions? Is this a normal feature of an upper receiver? This is a DPMS A3 upper. Is it normal for the bolt carrier to drag against the receiver like this? Sam |
Not until you put quite a few more rounds through the barrel.
Try it again with the barrel attached to the upper.
If the bolt was removed from the carrier and upper removed from the lower then I don't think it's the carrier rubbing, rubbing is usually from the cam pin. I think the gas tube is misaligned and striking the carrier. what kind of stock? are the buffer and spring correct for it? |
I'll try that.
If the bolt was removed from the carrier and upper removed from the lower then I don't think it's the carrier rubbing, rubbing is usually from the cam pin. I think the gas tube is misaligned and striking the carrier. what kind of stock? are the buffer and spring correct for it? The rubbing is taking place with the barrel removed from the upper receiver. The gas tube is also not present. The shiny spots on the upper receiver seem to correspond to the bolt carrier. The shiny spots are on the left side of the receiver, opposite the ejection port. Also, the rubbing takes place with the bolt removed from the bolt carrier. At this point, I think the most likely explanations are: a) the Les Baer bolt carrier is slightly over-sized b) the DPMS upper receiver has a slightly under-sized carrier bore c) the DPMS upper has a manufacturing defect ( the ledge I mentioned ). Thoughts? Thanks again. Sam |
|
Have you tried adjusting the buffer to up the tension? This might help to seat the bolt properly, and get you closer to a solution. I know the use of those buffers isn't too common, and the troubleshooting steps used in this thread most likely haven't taken into account that you have an adjustable buffer. To make life easier, here's more info on the buffer in question Oly buffer |
|
Yeah, I tried to adjust it today. I couldn't tell much difference. I'm guessing screwing the plug in will increase the pressure of the cylinder. I took it to the range today, and it seem to be behaving a little better. It failed to go fully into battery about 3 times in 20 rounds. Before it was between 7 and 10 failures in 20 rounds. Sam |
+1 Are the shiny spots at the rear of the cam pin clearance cut? You shouldn't feel any binding between the carrier and the upper if the bolt and barrel are removed. |
|
Not sure where the cam pin clearance cut is, unless that the semi-circular cut out at the top of the carrier bore near the front of the receiver. The resistance does take place when the barrel and bolt are removed from the upper. The bolt carrier seems to rub on a small ledge in the upper receiver across from the ejection port, about a half an inch back from the front of the ejection port. I'm beginning to suspect it's a combination of an improperly adjusted pneumatic recoil buffer, and that small ledge on the inside of the upper receiver. I wish I could figure out how to post a picture show I could show you what I'm talking about. Thanks. Sam |
 photo courtesty of MRW. the raised rectangular area on the left side of the upper. wear on the inside is normal but not optimal. |
AR Sponsor