View Full Version : Colt AR15 parts not interchangable?

March 6, 2011, 09:27 AM
Shooting with a large group of people yesterday and got talking with one guy who was telling me he in interested in an AR. He then told me what he wanted to build and said colts AR parts are not interchangeable with any other brand available? Is this true? For ex. not able to put a colt upper on a Bushmaster lower and vice versa, along with any brand other than colt on colt?

March 6, 2011, 11:10 AM
Most parts are completely interchangeable.

Colt on some models has used the large pivot pin on the front of the receivers to keep people from putting a true Military Spec upper on a Colt. Everybody else uses small pins on their receivers.

There are off-set adapter pins to use small pin uppers on Colt lowers.

Short story....you get more for your money in anything but a Colt.

Some AR Heads are of the Colt only mentality...get past that and buy the brand that offers the features you seek...and have a good time joining the AR club !

March 6, 2011, 11:40 AM

Most parts ARE interchangable. But there are certain things you need to look out for. DPMS is the most common interchangealbe parts you can find. Colt is the least interchangealbe out there. When it comes to the butt stocks there are two sizes "Commercial" and "Mil-Spec" and they do not interchange, you need to be sure you are ordering the right size. If memory serves me right, Armilite and Colt make uppers that can only fit on the own lowers or vis versa. Not every upper will fit on their lowers.

Some mfgs like Bushmaster use different size openings in the Carbon Fiber uppers and need an adaptor to mount standard size barrels.

Rails come in different configuations and may be a problem if they are Weaver type and you are trying to mount a pik type accessory.

If you take a little care and research your parts out first for size, you should have no problems. But, do the research.


March 6, 2011, 12:15 PM
Heres what doesn't interchange: Rifle buffer parts with carbine, commercial vs. milspec diameter tubes, and gas length tubes, carbine, mid, intermediate, rifle.

I built an AR from the following: AGP lower, LAR A3 upper, ARP barrel, ITS bolt, M16 nitrided BCG, mid length gas, rifle handguards, GI FSB, M4 Colt carry handle rear sight to cut down as a BUIS.

The LPK was Stag, the A1 Cav Arms stock got a Brownells rifle buffer tube kit, and the stripped upper a Brownells High Standard kit. I swapped the M4 delta ring for an early flat A-nothing ring. The flash hider will likely be Black River Tactical. Did I mention the Tango Down Battlegrip?

About a dozen brands there, what's not interchangeable is function specific.

If you don't see some popular brands in this build, well, observe the masses, and do the opposite. It's a 6.8SPCII in Foliage Green. Do the same thing in black, with a completely different supplier for each listed, and you get the same thing, an AR15 bullet launcher.

That would be two dozen different brands, same result. Pics are everywhere, we interchange a lot on these platforms. Why, some even buy Colt and take off parts to improve them!

Double Naught Spy
March 6, 2011, 12:35 PM
Interchangability hasn't really ever been a problem for me and my Colts. I have a couple Colts, Rock River, LMT, etc. I think either 2 or 3 pins in the Colts don't interchange with the others.

The pins weren't ever an issue until I took an armorer's course, spilled my tray of parts, and had to figure out what pins I needed to replace. So it took me another 30 seconds or so to figure out that I had picked up the wrong replacement pin and to try another.

I haven't ever had any break or otherwise be a problem. So if you don't strip down your lower completely and then spill the parts, chancs are, you won't ever need to know or care if your pins are a different size.

I haven't had a problem with interchanging uppers either and have switched them around in trying to come up with configurations best suited to my needs.

March 6, 2011, 05:37 PM
There's two areas where Colt has used nonstandard parts. First is the pivot pin as oldcspsarge pointed out. However, it has been some years since they have done that. Second is the fire control group pins. Colt has been using larger pins than the usual spec (in order to prevent someone from putting full auto FCG parts in a semiauto lower). It was not that long ago when they finally quit doing that.

With current production Colt rifles, they are "in spec" like everybody else. There are some parts here and there that don't interchange, but they aren't a result of the Colt pony (like the buttstock parts).

March 7, 2011, 12:32 AM
Above it was mentioned that ArmaLite uppers and lowers are not compatible with other brands but that's not true; they do fit other standard receivers. I know for a fact that an ArmaLite upper fits S&W and Charles Daly lowers and Daniel Defense and Bushmaster uppers fit an ArmaLite lower.