Problems feeding hollow points in a large frame AR

[LRPV]

I put together a large frame AR chambered in 6.5 Creedmoor. Mega receivers, Krieger barrel, JP BCG, and using DPMS 10 round and Magpul 20 round magazines. I shot factory Hornady 140 Amax ammo through it without any problems but just recently starting loading for it myself. Using Berger 140 hybrids, the bullet crashes in to the barrel and won't chamber from the magazine. Test rounds shot well by single feeding, but that was not my intended purpose. The 140 Amax and 123 Amax will chamber just fine evidently because the sharp tip allows them to center up in the slight bevel at the beginning of the chamber.

Just wondering if there is something I am over looking that is simple or it looks to me like I need to have little larger "cone" at the chamber.

Jason

 

[DR1586]

The weak link is typically the magazine on any semi auto. I would suggest loading a dummy AMAX round and chamber it out of the mag, then extract it. If the tip is messed up then all the tip is doing is deflecting off what the burgers are hitting. I would try a different magazine if you have one available. Check your seating depth to make sure the bullet isn't clipping the front of the mag. From there you may need some work on the feed ramps.

You're not seeing any cases of bolt override are you? How hot is the load?

 

[LRPV]

I only have the two types of magazines, DPMS and Magpul and they both do it. I started around 2.805 COAL and set the deeper from there. The Amax tipped doesn't look to be messed up after chambering one. I started out at 37 grains of H4350 with the 140 hybrid but it isn't a cycling issue, it won't even feed the first one from the mag. The feed ramps look like the M4 cuts on smaller Ar's and that may be the issue as there isn't enough material to kick the nose of the bullet up enough to get in the chamber. At least that's what it looks like to me, and it isn't that much at all but it sure does keep it from working.

 

[mao0720]

I had the same issue with a 260 rem AR that I built. The issue is that high bc bullets have a very long ogive, the pin of the bullet is so narrow that it will go between the bolt lug notch in the feed ramp. On mine I was able to modify the feed lips of the magazine to give the rounds a more point up attitude. This let the rounds essentially go straight into the chamber instead of riding on the feed ramp. This and a single round loading block were the only things that let me use higher bc bullets.

 

[LRPV]

I had the same issue with a 260 rem AR that I built. The issue is that high bc bullets have a very long ogive, the pin of the bullet is so narrow that it will go between the bolt lug notch in the feed ramp. On mine I was able to modify the feed lips of the magazine to give the rounds a more point up attitude. This let the rounds essentially go straight into the chamber instead of riding on the feed ramp. This and a single round loading block were the only things that let me use higher bc bullets.

I think you are spot on. That's the only way I can see to fix it.

 

[mac86951]

I only have the two types of magazines, DPMS and Magpul and they both do it. I started around 2.805 COAL and set the deeper from there. The Amax tipped doesn't look to be messed up after chambering one. I started out at 37 grains of H4350 with the 140 hybrid but it isn't a cycling issue, it won't even feed the first one from the mag. The feed ramps look like the M4 cuts on smaller Ar's and that may be the issue as there isn't enough material to kick the nose of the bullet up enough to get in the chamber. At least that's what it looks like to me, and it isn't that much at all but it sure does keep it from working.

My 6.5CM load is 39.5 H4350 with a Sierra MK 142gr at 2.780" No trouble with the DPMS upper, which has nicely polished M4 dimples in the receiver. Run a bullet tip across those M4 cuts, you might have a barrel extension that is protruding slightly making a shelf that catches on the hollow point.

-Mac

 

[LRPV]

Mac, the bullets slide right through the extension with no resistance, they just need to be kicked up slightly and they would chamber. The way it is they just slide right into the end of the barrel and jam up.

 

[mac86951]

Mac, the bullets slide right through the extension with no resistance, they just need to be kicked up slightly and they would chamber. The way it is they just slide right into the end of the barrel and jam up.

Hmm, that gets me wondering...

If i sight along a feed ramp the angles lead right to the center of the chamber.

Additionally, the mouth of the chamber (proper term unknown to me) where the case head will reside with a chambered round has a slight bevel to it.

I've seen places that cut the barrel extension instead of the barrel to head-space, and I'm sure your barrel + extension with your bolt meets head-space requirements (right?), that means my approx. measurement between the barrel mouth and the extension is ~ .700" (rough measurement since I didn't remove the barrel from the upper for caliper depth measurement) I'd estimate my error at +/- 0.075" but given that slop in measurement, sighting along a feed-ramp it would still point near the bore. Does yours have a different angle that puts the end of the barrel in the way?

-Mac

 

[LRPV]

I don't know if there are more headspace requirements other than the relationship between a chamber, go gauge, and the bolt. The go gauge fit with the bolt, and would not lock up with two pieces of scotch tape. Other than that, I am not aware of the other specs. Near as I can measure without taking the barrel off I have around .660" from the end of the barrel to the end of the extension. That is as far as the round will "chamber" as is.

 

[chkunz]

I have had this problem on a couple of different ARs and the problem turned out to be short cycling. If you do not know what that is or how to test for it maybe we can help. It is easy to check.

 

[mac86951]

Interesting. I think I may have it. Again, I'm back to my DPMS 6.5 creedmoor as my reference so I'm not a know-it-all, but I do have a LOT of AR experience (mostly 15s)

From looking at your photos your feed ramps are way wider than mine, and at a shallower angle.

You might be able to fix the issue with magazine lips, but that seems awfully far on feeding to fix from a Magazine angle, similarly removing more metal from the extension I would estimate to make the problem worse.

Does it do this from both feed ramps? I'm guessing from your previous comments that it does. I'd recommend a new barrel extension be fitted with that picture and a dummy round along with the request.

Best Guess, I'm thinking someone polished out a barrel extension for a .308 to load match rounds without scratches and installed it on your barrel with the idea that it'll work fine. Guess tolerances overlapped a little to far on this one.


At the same distance from the magazine, my 6.5 has the bullet tip near bore center-line. I'm loading 2.780" with a 142 SMK.

I'd also recommend a no-go gauge to be sure. it'll be a little more accurate than scotch tape, however, in this case too much head-space certainly isn't the issue, or I'd expect feeding to not be a problem.

Try a dummy (no powder or primer), or loaded round with 140gr, 37gr H4350 (not near maximum, actually that is way on the lower side of pressure my shooting round is @ 40.2gr) 2.780" COL. Its possible those 0.020" are the issue too.

-Mac

 

[mao0720]

The difference is that the 142 SMK has a very round, tangent, ogive so it rides the ramp easier that say the berger hybrids with their secant ogive (mostly) that the OP is using. I doubt if anything is wrong with the feed ramps or bolt lug cuts, it an issue of trying to use long, skinny, secant ogive bullets in an AR platform.