AR denting brass when feeding

stealthman

New member
I have kind of a weird issue with my new (used) upper. Upper is Dtech in .204, lower is Plum Crazy, mag is 20 round PMag. When I charge the gun and with every new round that feeds into it, it puts a dent about 1/4 of the way down past the neck and makes light a v-shaped scratch on the actual bullet. The dents vary in severity, but they are horizontal on the brass and between 1/16 and 1/4 inch long. When I charge it slowly, it looks almost like the feedramp is doing this. I suspect the magazine is not letting go of the round soon enough, but haven't had the time to get back out and try it with a different mag. Anyone experience this or have any ideas?
 
What you are seeing is one of the typical problems with a .204 in an AR. The .204 case is longer than a .223/5.56 case, so if everything is not perfect, you can get exactly what is happening to you right now.

An AR is made up of many pieces: Upper receiver, lower receiver, magazine and more. The upper receiver can function perfectly on another lower, with a different magazine, but put that upper on a different lower, and you may end up with issues. The magazine can have a huge role in it as well.

I have tried P-Mags in the shop, and where I like them for most purposes, I have not had good luck with them while feeding the .204.

The way the magazine is held in the lower receiver also has much to do with how the .204 will feed. First off, ditch the P-Mags and try a military surplus magazine. If that doesn't take care of it, see if you can try a friend's lower receiver, along with a different magazine.

What causes the issue is the fact that the .204 case is longer. When the case rim is not clear of the magazine and the tip of the bullet strikes the top of the chamber, the bolt tries to bend the case over the chamber mouth. Shorter OAL if you are hand loading can help as well.
 
I'm 90 percent sure it's the magazine. Those Pmags keep the rounds firmly on each side of the mag, where the mil-spec style lets them come from the center more. When I charge it slowly, I can see right when the issue occurs and it looks like the mag is causing it. I think I will try one of my mil-specs and see how that works. Might try cutting back the Pmag lips a bit at a time to see if that helps too. Thanks for the explanation Mike. On the positive side, I have only shot factory ammo through this (HSM and Hornady) and it absolutely shoots darts. I'm excited to work up a few homeloads to really find out what it likes.
 
You are probably right. I have never had a Plum Crazy lower on a .204 upper, but I had high hopes for the P-Mags, and they just didn't work well for the .204.

There are certain lowers that are more problematic with the .204 than others, but I just don't have any experience with your lower.
 
Originally Posted By: doggin coyotesMike,

Are ARs chambered in .223AI known for having feeding problems and dented cases when being chambered?

As I had touched on earlier, the crescent dent in the side of the case can actually be caused by many things: How the magazine is held in the lower receiver (this is different slightly from brand to brand). The magazine itself, and the COL of the loaded round.

The picture I was trying to paint is that the cartridge is slightly longer than the .223/5.56, and this is where the problem stems from. When machining the barrel-shank for a .204 barrel, the chamber-mouth has to be formed differently than a standard .223/5.56. If the shank were formed the same as a standard AR barrel, it is likely you would have problems with any lower/magazine combination. Getting the chamber-mouth constructed correctly will allow correct feeding provided everything else is attended to. There is a limit to what can be done with the chamber-mouth because the chamber has to support the area above the case-head. If it weren't for that limitation, .204's could be produced to feed/function with any magazine/lower receiver combinations.

That being said, I would expect you would have to take the same steps to correctly feed a .223 AI. The COL and case design would make me believe you would encounter similar issues.
 
I tried a C-Products and a Bushmaster mag. The C-products mag did help. It now only dents the brass feeding from the right side of the stack. The brass was also catching at the neck on the front of the mag, so I polished that down just enough for it to clear. I think I will take it by my local gun shop and try a different lower or throw in a metal mag release and see if that is more secure.
 
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