View Full Version : A2 to A3 Conversion

04-19-2009, 12:20 AM
Hi folks,

I purchased this BCM complete upper receiver assembly in the hopes of replacing my A2 20" receiver. Will this be compatible even if the A2 barrel doesn't have M4 feed ramps?

This receiver has the M4 feedramps machined in. This is to be used with barrels that include the M4 barrel extension.

Hope that isn't a deal breaker :eek:


04-19-2009, 12:22 AM
You need a barrel with M4 feed ramps or reliability will be comprised.

04-19-2009, 1:13 AM
IIRC, it goes something like this.

M4 extension on M4 cut = Good
M4 extension on Rifle = Ok

Rifle extension on Rifle = Good
Rifle extension on M4 cut = bad

Blacktail 8541
04-19-2009, 6:49 AM
Randall will be along shortly with his pics to explain it to you. But basics are, if your reciever has m4 cuts , your barrel extention needs them.

04-19-2009, 1:26 PM
Do we generally want M4 feedramps or no when building an AR? I'm not sure I understand what they are for and when they are needed.

04-19-2009, 2:09 PM
Do we generally want M4 feedramps or no when building an AR? I'm not sure I understand what they are for and when they are needed.

On semi-auto rifles, it doesn't matter if you have them or not. The original M16s and M16A2s didn't have them. The CAR came along and they shortened the gas system, but the barrel length was short enough that pressure dropped off very rapidly so the cyclic rate was not effected greately. When the M4 came along, they shortened the gas system but left a substantial portion of barrel past the gas block and as a result the cyclic rate of the gun shot up quite a bit. It hit a point where the carrier was returning back to battery before the cartridge was completely seated at the top of the magazine. The result was the bullet would be hit high on it's centerline and would nose dive into the receiver instead of making contact with the barrel extension's feed ramps and the gun would jam. To combat this problem, extended feed ramps were ground into the upper recever and the extension's feed ramps were enlarged. The buffer was also changed to having tungsten inserts instead of steel and tungsten inserts and the H3 buffer was born.

Unless you can pull your trigger at the ~1,000 RPM needed to cause a similar jam, then it won't matter if you have the extended "M4" feed ramps or not. However, if you are pieceing your upper together you need to be careful as putting a standard barrel extension into an upper with M4 feed ramps because it will create a gap that can produce some VERY nasty jams when the bullet tips gets lodged in this gap and the carrier crushes the cartridge in the upper receiver (with, I'm sure, a potential for detonation). Having the extended feed ramps on the extension and NOT the upper just creates sharper edges and a less gentle transition which could bounce the round above the chamber and cause a jam, but it's MUCH better than the previously mentioned set-up. Having both with or both out will not help or hurt reliability in a semi-auto rifle.

Most companies make their uppers with either because in semi-auto rifle it really does not matter, but for the hundreds of thousands of legal full autos in the country, most will go for the feed ramps because of the increase in reliability.

04-19-2009, 5:28 PM
This should clear things up.

04-19-2009, 7:16 PM
Thx for the replies everyone. Looks like I bought the wrong one. Since you guys helped me out I'll offer this upper receiver for $95 + shipping if any of you guys want it before I create a WTS 2m.

04-19-2009, 10:21 PM
Thanks Neo, just what I was asking.