AR-15 Chamber Dimension Issue???

G

GunAddict1962

Inactive
Ran into a problem with my latest AR build. Bought a new 1-8 twist 20" DPMS varmint barrel from Midway. Everything came together fine. No obvious headspace issues. Barrel looks nice inside. Immediately noticed though that unfired rounds would stick in the chamber. Not every round, but about 2 out of every 10. Takes a good rap on the charging handle with a rubber mallet to move the BCG. On every ejected (unfired) round, on the bullet, about .05" above the lip of the case, there's a very distinct ring imprint. Noticeable on nearly every ejected round, significant on the stuck rounds. A crisp ring, with no visible land marks. No free bore in this chamber? Tried 5 brands of factory ammo 50-62gr and 3 different reloads 50-68gr with bullets at different seat depths. Nearly every type would eventually lodge tight in the chamber. It takes seating bullets way deeper than they normally would be seated to prevent this. No signs of dangerous pressure levels on spent brass, but really poor accuracy--2 inch groups at 100 yards. No consistency. Fliers every 3-4 shots. (In case you are wondering, I didn't really find the problem until just about finishing up on the range.) I'm not a smith, but I've been around guns a long time. Am I interpreting things right? An out of spec chamber/free bore?
 
I have a poor man's bore scope and I can see the start of the lands and grooves. They look good. But I can't really see the freebore area. Can't quite tell if the lands and grooves run all the way into the chamber of it there's a small ring of metal there.
 
I would say to cast the chamber, but I'm wondering if it might be hard to get it out to check what you have. Of course, you could put something in there as a release agent, to slick it up, then tap it out with a dowel rod, if need be, but it may release on its own. I'm speaking of using Cerosafe to do the casting. You would need to remove it as soon as it hardens, as it shrinks undersize then, and at 30 minutes, you measure it with a mic or caliper, as it is exact size then.

Cerosafe:

http://www.brownells.com/gunsmith-tools-supplies/barrel-blanks-tools/barrel-chamfering-accessories/cerrosafe-chamber-casting-alloy-prod384.aspx
 
Chamber dimensions are ground into the reamer.Poor technique can make a reamer cut oversize.A correct reamer will not cut an undersize chamber.
I do not have reamer prints to compare .223 and 5.56.
You can read a description of the different chamberings on Kreiger's web page.

The general idea,without numbers:You can shoot anything that will mag load in a 5.56 chamber.The 5.56 chamber is slightly looser for reliability and it has a little longer throat to accommodate longer bullets,such as 62 and 77 gr.

The .223 Wylde chamber is a little tighter for match accuracy that still works in an AR.It is throated to accommodate any match bullet that will fit in a magazine,plus a typical 80 gr bullet loaded over mag length for longer stages.

The original commercial .223 chamber was ideal for(I'm guessing) the bullets that would stabilize with the typical 1 in 12 twist.It will vary with bullet design,but I'm talking 55 gr and the lighter varmint bullets.

The short throat ,which can cause pressure problems with 5.56 ammo,would typically be used on a 1 in 12twist.

It does not make much sense to put a short throat .223 chamber in a 1 in 8 or 1 in 7 twist.

That does not mean it can't happen.

Perhaps Joe Production went to work at DPMS,got his dispatch sheet,and it told him work order 213678 was his next job.Chambering 40 barrels,all just like yours.

Joe goes to the tool crib,checks out his reamers and gages.

Through a bad reamer or missed operation,and maybe checking boxes without actually gaging his work,an out of spec barrel was likely shipped.

Lucky you!!

Call DPMS and talk to them.You may be able to send the complete upper.

You might learn something with Silly Putty.Mark a cleaning rod for length so you can put a patch on a jag,run it in from the muzzle,and leave it just ahead of the throat for a cork.Unscrew the rod.

Take a gob of silly putty ,enough volume to fill the throat and forward part of the chamber,roll it to a snake and drop it in the chamber.

Stand it in the corner a few hours.muzzle down.The silly putty will flow to pick up every tiny detail.

Then use the rod to push it out.No,you can't mic it,but you can see detail,and eyeball with calipers approx. lengths.Silly putty can tell you things.

If you have a friendly dentist,I used to use a Co-Flex brand 2 part dental impression compound to makea cast,like Cerro-Safe,but a soft rubber.Amazing detail!
Of course,be sure to clear everything out of your barrel before you shoot again.
 
While you say there's no obvious headspace issue, has it been headspaced?

How does it do with the go gauge?
Then how does it go with the no-go?
 
If you can't chamber factory spec ammo without it sticking, just send it back rather than trying to figure out what is wrong.

No amount of figuring will solve the problem, and anything you do will ruin the chances of it simply being replaced with a good barrel
 
You must log in or register to reply here.
Back
Top