Differences between AR-15 lower receivers?


June 29, 2005, 11:55 PM
I know Colt is the only one with nonstandard pins. I understand manufacturers deal with the autosear area differently, with bridges, blocks, or shelves. Is there some chart out there that shows the differences between manufacturers, maybe with pictures? Do these interfere with trigger installation?

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June 30, 2005, 12:02 AM
I haven't heard any major incompatibility problems (but that doesn't mean they don't exist). I usually hear arguments on cast vs forged processes when it comes to selecting a quality lower receiver. I believe ARFcom has an informal pictoral compilation on all the different brands of lower receivers, but I don't recall seeing a discussion on exactly what differences exist between each. I know the major brands seem to be compatible from peoples' experiences: Bushmaster, RRA, Olympic, RRA, DPMS, etc.

There might be some slack between the upper and lower receiver fit that makes it a tad loose with mixed brands. They make an "accuwedge" specifically to cure that. I know my purebred Bushmaster has the slightest bit of play if you wiggle the upper and lower receivers in opposite directions. I checked all the Bushmasters and Colts at the gunstore before I purchased mine and they all have a minute degree of wiggle, but its not enough to worry over. Eliminating 100% of the play would likely make it almost difficult, if not impossible to assemble and disassemble the upper/lowers.

However, if you go look up the aforementioned pictoral thread, I don't think you'll see too much physical difference between the lower receivers. I would fathom most of the differences are on the scale of hundredths and thousandths of an inch.

June 30, 2005, 07:23 AM
I agree with Cesiumsponge.

As far as I know Colt is the only one with a sear block or full with web to prevent installation of an autosear.

As far as I know the high shelf and low shelf recievers have no problems with any of the semi trigger assemblies out there, be they match or single stage.

Some of the high shelf recievers have problems with things like the lightning link. But if you can afford the $6000 for the RDIAS then a new low shelf lower would be a drop on the bucket!

June 30, 2005, 08:04 AM
the $10000 for the RDIAS then a new low shelf lower would be a drop on the bucket!

There, I fixed it for you. :D

June 30, 2005, 08:13 AM

I stopped keeping track when I became painfully apparant that my income would never grow fast enough to be able to afford one.

Damned Hughes Amendment :fire:

June 30, 2005, 11:50 AM
of gun cost means anything to you? Hell, that's just 5000 rds of 7.62 x39 ammo, which is just a year's fun (at most).

June 30, 2005, 01:05 PM
Taking Colt out of the mix, Just about all of the "brand name" forged lowers on the market will work just fine but there are some very minor differences.

First off, I recommend staying away from Cast lowers. I know many people have them and haven't had any issues but the fact remains they are a lot more fragile than their forged counterparts and generally only cost $20-$30 less. Not enough of a savings IMO to justify the added risk.

As stated, some have a "low shelf" (LAR made lowers) and some have a "High shelf" (ex. CMT and LMT). Low shelf lowers will accept an RDAIS and high shelf lowers will not but unless you already own an RDAIS I wouldn't worry about that.

There is really only a handful of machine shops milling lowers. LAR (who makes BM "L" series, DPMS, Grizzly and Ameetec) CMT (who makes RRA and Stag and some reports claim they make Colt's as well?) and LMT (who makes LMT, PWA, Lauer and a couple of others that I can't think of right now). There are probably a few other machine shops but those three are the best known.

Keep in mind though that even though they may make lowers for a number of different companies, they may not all be made to the same specs. LAR is a good example. The lowers they make for BM are just Hard Coat Anodized. The lowers they make for DPMS and Ameetec are Teflon coated after they are anodized.

Which brings up another point to be aware of. Some people like Teflon coated lowers because they feel they are more wear resistant. Some people don't like them because they usually will not match an Anodized upper real well (color wise). Either way, know that if you plan to paint your lower, none of the typical products used will stick to a Teflon coating very well.

As far as upper\lower fit. That is always a crap shoot. Even if you buy your upper and lower from the same source at the same time, they may not lock up tight. Functionally it does not matter in the least. A 100% tight lock up offers absolutely no advantage over a completely loose combo accuracy and performance wise.

If you want tight, go with a CMT milled lower as they mill the rear take-down area a little fatter than most. It is done on purpose to help tighten up Upper\lower fit but just know if you get an upper with lugs a little fatter than norm, you may need to file the upper's lugs to make them fit correctly. The other solutions to tighen up the Upper-lower fit is either an Accuwedge or a #7 o-ring around the front lug.

I own (or have owned) the following lowers:
Colt (?)
Eagle Arms. (?)

While I love the Colt (if for no other reason then it's the only one actually marked AR15) but I would have to say, out of that group, The RRA's are my favorite because of their fit\finish\quality\cost.

Just my $.02... YMMV

June 30, 2005, 09:39 PM
Thank you! That was very informative.

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