AR15 Gurus

Dan A

New member
I just want to make sure I understood correctly please:

I have some AR15's, all in 556.

I am wanting to turn one into a .204.

I understand that I just buy a .204 upper but ...

1.) Do I need different magazines or will the pmags for my 556 work?
2.) Do I need a different bolt carrier group or will the ones I have for 556 work fine?
3.) Is there anything else that has to be changed?

Thanks for confirmation/help.

From my research it seems I do need different mags but the BCG should work.
 
All you need to swap is the barrel.

223 bolt will work fine.

When I had a 204 AR, I opted to use standard metal 223 magazines over the polymer Magpul magazines and had no issues.
 
I have 2. The last one I put together myself with a Wilson barrel and seekins adjustable gas block. Nice to be able to get the gas dialed in just right.
I didnt have any luck with pmags they are too tight on the shoulder. Standard metal ones work but I have found the Lancer mags to work the best. I use the clear ones and love them for checking how many rounds I have left, like after a stand that didnt quite go right lol.
 
204s are fun. I prefer the ASC stainless mags. They just seem to load easier. Also you might want


to go to a heavier buffer like an H3 but that's open to experimentation.
 
I run ASC STAINLESS magazines. I emphasize STAINLESS because you can load out to 2.316 nominal vs a regular GI type at 2.260. The prices are very reasonable and they are coming out in colors so that is a benefit if you run various 223 head sized cases.
also.

Greg
 
I'm glad I read this thread as I hadn't heard of ASC before. Some very nice stuff there !
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I've had the normal G.I. type metal mags work best in my 204 ar's as the others have mentioned. I like adjustable gas blocks on mine too.
 
Short of playing with the gas system/buffer system, the only thing you need to change is the barrel. The .204 is based on the .222 and .223 family of cartridges and will feed from a standard STANAG pattern magazine so the mags you have will work as well as the bolt carrier group. IIRC the .204 is a little higher pressure so you might need a heavier buffer.

Carbine buffer (3 OZ)
H1 (3.8 OZ)
H2 (4.5 OZ)
H3 (5.0 OZ)

A standard 16" AR usually has a carbine buffer (3 Ounces) but an H1 buffer is better for most carbines since it slows them down and most are over gased.

Carbine buffers work best with mid-length 16" rifles.

18" rifle length gas systems arent usually picky but you may have to experiment. Mine I believe is running a carbine buffer.

Try your stock buffer and as long as ejection is close to 3 oclock, you are good. Forward of 3:00 is okay but indicated its got more gas/more bolt velocity than needed. Behind 3:00 (closer to 4 or 5) indicated the bolt velocity is too slow and the rifle may be undergased.
 
Originally Posted By: Dan AI just want to make sure I understood correctly please:

I have some AR15's, all in 556.

I am wanting to turn one into a .204.

I understand that I just buy a .204 upper but ...

1.) Do I need different magazines or will the pmags for my 556 work?


Thanks for confirmation/help.

From my research it seems I do need different mags but the BCG should work.

you're right that standard pmags will not work with the 204. the shoulder is placed differently (further forward) than a 223/556 and the rib on the sides of the pmag wont line up right and cause them to bind.

you might get 3 or 4 rounds in before they start to bind up.. but they will.

as mentioned standard metal mags work just fine though.
 
When I had my 204, I used the straight 20 round Magpul Pmags with ZERO issues. Typically I wouldn't load more then 10-15 rounds at a time because this was a hunting gun.
 
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