AR-15 parts compatibility
 

I've been poking around looking at AR-15s. Their are tons of choices. For a rifle to be labeled an AR-15, does it have to have certain common characteristics that will make it 100% compatible with other brands parts?

If I bought a Bushmaster (this is just an example) rifle, would I later be able to change the forestock, barrel, etc...using parts of from other manufactures?

It seems like some of the modestly prices AR-15 clones are not actually AR-15s.

Educate me.

I think they are all mostly standard now. Some older Colts are not compatible with "standard" parts, but that's about it.

I also believe that Rock River specifies their own tolerances on their internal parts, so there may be compatibility issues there too.

Most AR-15s are AR-15s.
The .22LR based models aren't always.
The biggest desrepancy can come in on AR-10 (.308) component compatibility. Either a DPMS pattern or Armalite pattern. In AR-15's things are pretty much standard.
Edit: to claify Legion's post, some of the older Colts had larger hammer and trigger pins than what is common now.
Edit again: Modestly-priced - it is possible to assemble a modestly-priced, functional AR-15 using a Parts kit ($425-500) and a lower receiver ($70ish). Then build on that platform. i.e. rails systems, optics, foregrips, etc...

Yes, it's just about all compatible after the upper and lower receivers, which are sometimes caliber-specific, and the barrels are, obviously, always caliber-specific. A few years ago I bought a Spike's stripped lower receiver, a RRA lower parts kit and then all the other parts that I wanted on the lower. Then I bought a complete upper from Del-Ton, some Magpul handguards and it all fit together like a Swiss watch. Shoots as well as my out of the box Bushmaster ever did.

Big difference is recoil buffer length between milspec and civvy, and full length vs. collaspable.

Yeah, it's actually the diameter of the tube that determines mil-spec vs. commercial.
Collapsable vs. A2 is length.

Dan's on the money.

Buffer tube: diameter, civ vs. mil
Buffer and recoil spring: length, carbine vs rifle
Hammer/trigger pins: diameter, "Early" Colt vs. rest of the universe

That's it.

Now, there are some other considerations if you start playing around with nonstandard parts. For example, an upper receiver that is sold/described as "reinforced" may not have sufficient clearance for any bolt catch other than the standard one (ruling out accessories like a "tactical bolt catch" or the Magpul BAD lever). But these are special cases.

The whole .308 thing is a giant can of worms. I personally stick with the KAC (and DPMS) "standard" because KAC charges way too much money to ever go out of business. :D

some barrels require different sized gasblocks..

Yes, but unless you're building your own upper, you don't need to worry about that. I would guess the vast number of barrels sold have already been assembled into an upper.

Besides, it's easy to find another gas block from any number of sources if you happen to have purchased the wrong size.

Absolutely. That's what makes an AR-15 platform so desireable. You can design a configuration to suit nearly any purpose. 3-gun speed matches, long-range taget-shooting / prairie dog hunting, large caliber pig-hunting, you name it, by simply switching top-ends. I like the most inexpensive forged lower with a RRA two-stage trigger, and a number of tops.

That RRA NM trigger is the best bang-for-the-buck you can get when modding your AR.

I took a 3-day rifle class with a buddy of mine. I brought my .308 AR and he brought his M4 clone. Day 1 he was shooting around 3-4 MOA. That night I installed my spare RRA NM trigger in his gun (all of 2 minutes) and the next morning he was shooting 1.5 MOA. Literally an overnight 50%+ reduction in group size, and for something like 120 bucks.

Well it's something to lookout for if you wanna replace the one on yer upper with one you buy online.. Better to measure before you end up with the wrong piece..
Most will be standard, but some aren't.. Takes 5 seconds to measure.

I built my AR with a 36 year old receiver, (pre-ban), on a Stag Arms upper. I did have some fit issues with the lower kit but the fit from upper to lower is very very tight. Even though there is a difference in civy & gov. recoil buffer tubes they fit any as long as you have the right stock. Regardless of make.

Technically speaking no one makes an AR15 but Colt. But that's splitting hairs. I was recently told by a local gun store that you cannot mix uppers and lowers when it comes to RRA stuff. I know this to not be true. I built a Spike's lower/RRA upper a couple years ago and it turned out fine.

My latest build, All Rock River, just waiting on the barrel. Only problem is, it is on indefinite back order. Placed order in July, no barrel in sight.

http://i909.photobucket.com/albums/a...3/IMAG0827.jpg
http://i909.photobucket.com/albums/a...3/IMAG0829.jpg

What barrel are you looking for?
I've been able to come up with them fairly quickly?

I ordered this one in 16".

http://www.rockriverarms.com/images/...ts/bblss12.gif

The Wylde chamber is probably the hold-up. Most mfgrs do things in batches. I've been waiting months for a 20" HBAR in 6.5 Grendel from Alexander Arms.

I managed to get an order in w/ Black Hole Weaponry for a 20" HBAR in .264 LBC-AR (basically the same chamber) right when they were starting a run of .264. Had the barrel in a few weeks.

I agree. They're running the heck out of NATO to keep up with demand.

Several things no one has mentioned yet. First is chambering, .223 vs 5.56 can be an issue. For an accuracy gun .223 has a tighter chamber while 5.56 is the NATO size and can generally shoot anything. Second is shooting 22LR ammo which is really great as it saves a ton of $$$$$ and lets the wife and children also shoot. I would suggest a dedicated 22LR upper as the barrel twist in a "bolt adapter" type of conversion is wrong for rim fire ammo and you will be lucky to hit anything. There are some that can use the center fire trigger and some that do not. Third, I'd suggest buying a pre-built upper and use it to learn on. Saves time and stress and they generally work fine unless you plan to shoot wood chucks at 500 YDs or so. If you do that get one that has a full length rail on top so the optics are easier to mount. Last of all make sure the scope/red dot/etc you get has a mount that is sized for the rail and the scope is mounted correctly with a mount kit. I see at least one a month at our range that either come off or loose after a mag of shots as they are not mounted correctly.

Good luck as they are fun to shoot although I sold mine to support my black powder habit!

Colt MT6400R. Me likes.

http://www.coltsmfg.com/Portals/0/pr...12/MT6400R.png