LR308 Upper/Lower Compatibility

[one918]

I have assembled AR15's but this will be my first .308 build. I chose a TM lower and recently a Aero m5 upper, both are DPMS. The fit is not cosmetically pleasing by any means and TM has not produced their uppers for some time now as far as I know for a better fit, but I did not think it would be as noticeable.

My questions, is the fit between brands usually this drastic? Thanks in advance for any input.

[baih777]

Yes the fit can be that drastic. 308 AR does not have a standard. TM is standard first gen DPMS. AERO is DPMS pattern internally but not external. Now you will end up with two.

[one918]

Thanks much for the quick reply. Can first gen DPMS uppers be found easily?

[Dr69er]

You may also have another Issue that being the takedown and pivot
pins may not be the same diameter and/or the correct length...

The 308 Aero's do not follow the Gen 1 DPMS pattern very strictly by
any means, and complicating the Issues is that there is not a strict
Mil-Spec standard for 308 pattern AR style rifles.

Good luck.

[Drew Eckhardt]

My questions, is the fit between brands usually this drastic? Thanks in advance for any input.

Yes, and coming from an AR15 background where parts swap like Lego bricks that may not be your only surprise.

My "LR308 compatible" Aero Precision M5E1 upper will not close with my genuine DPMS LR308 lower because the radius at the rear doesn't match.

The charging handle on my Alex Pro (Fulton Armory sells the same receivers, and probably others) DPMS "high" receiver won't clear the buffer tower on my Aero Precision M5 lower.

After four tries I gave up trying to find a non-DPMS (those were out of stock due to the California and 2016 election panics) upper for my DPMS lower because I didn't think the slop was acceptable for a $2K gun.

Apart from Knight's Armalite pattern M110 and now HK's 417 derived (G28E to pick nits) M110A1 there is no milspec large-frame AR and you can't get blueprints. Everybody reverse engineers either the DPMS LR308 or Armalite AR10.

If I wasn't dealing with a grandfathered large-frame AR assault weapon (it needed to be a featured semi-automatic bullet buttoned rifle some time before December 31st, 2016 but didn't need to be in that configuration Midnight January 1, 2017) and didn't have a matched upper/lower set, I'd sell what I had and start over.

Figure out what hand guard you want. They come in proprietary plus 2.5 flavors - high (post-2009 DPMS) and low (pre-2009 DPMS, Aero Precision are made this way) with low available in DPMS and Armalite barrel nut threads. Some vendors (Odin works) have adapters so their handguard top rail lines up with either height, some (Midwest Industries) make handguards both ways, some DPMS low rail fore ends (Aero Precision) have both flavors of barrel nuts available. Some proprietary interface handguards (Aero Precision) have barrel nuts for conventional uppers (Armalite and DPMS low in that case).

Supposedly the proprietary interfaces allow a stiffer connection. The upper + handguard + barrel nut combination may weigh less, although a scoped large-frame AR will still be a heavy gun after losing a couple ounces. If you're aiming for light weight start with a DPMS G2 or other small-frame .308 with AR15 diameter bolt carrier and dimensions aft of the magazine well for a half pound less metal between bolt carrier and receivers.

Buy a matched receiver set (or upper) to go with it, where you might have a preference for no forward assist, DPMS style at the shell deflector, or Aero Precision at the rear. Matched is preferred over separately sold receivers from the same vendor because the pair may be made for a tighter fit. If you want a functional forward assist, your bolt carrier needs teeth matching the receiver button location.

Either DPMS upper height should mate to any LR308 lower although that wasn't the case with my Aero Precision M5.

Mechanically, mismatched low and high DPMS uppers and handguards mate but the top rails won't line up.

Some handguards may require some Dremel work to mate with some uppers - Aero Precision "wings" at the receiver end of their handguards do not clear some uppers.

Some vendors like Aero Precision and Mega do a handguard / upper / lower combination with everything working together. Aero Precision's Builder Kits (mlok or keymod, 12" rifle length or 15") are good and priced well - there are no growths beneath the barrel nut that would be less comfortable grasping the forearm as you would for standing high power.

Like AR15s, some uppers come with M4 feed ramps and some do not. I don't know what the situation is with barrel extensions.

With no milspec you should check head space once you have your bolt and barrel.

Note that some vendors (Aero Precision) have smaller firing pin holes in their bolts to avoid problems with 6.5 Creedmoor.

Pay attention to the LPK differences. The bolt catch, bolt catch pivot (threaded, often included with the receiver), pivot pin, and take-down pin are different than the AR15. You can buy conversion kits or separate parts if you have a LPK you want to use.

An AR15 magazine catch usually works but the receiver is wider. You may want a longer (Aero Precision) button for featureless or catch with longer shaft (Armalite). The Raddlock convertible bullet button instructions suggest using that catch.

The Aero Precision pivot pin is a little longer than the standard DPMS LR-308 part. The DPMS part supposedly works but looks worse because it's not flush.

Standard AR15 fire control groups work, except those with lightened hammer springs may have problems with hard 308 primers. Timney sells an AR10 version of their triggers. Geissele uses full-power springs with the pull reduction coming from sear geometry and finish.

Note that large-frame ARs like the LR-308 are 5/8" longer behind the pistol grip than AR15s which may steer you towards an A1 stock if you planned on using an A2.

Have fun, and enjoy the learning experience!

[Drew Eckhardt]

Thanks much for the quick reply. Can first gen DPMS uppers be found easily?

Every DPMS branded or compatible upper you can buy is "first generation," but may be low (pre-2009) or high (post-2009) tang.

DPMS does not sell G2 uppers, with the G2 being their "small frame" .308 with AR15 bolt carrier diameter and length aft of the magazine well (the LR-308 is 5/8" longer) which knocks a half pound off the weight.

[PaperPuncher]

My buddy just had an issue fitting a Palmetto upper on a TM lower so I let him put my Aero M5 upper on the TM to trouble shoot the issue. For what it is worth, they fit together and everything functioned fine. Fit was not very pretty though.

[baih777]

here are pics of my TM lower with a DPMS upper. color match is way off, but I didn't care. it fits and the rifle is a blast to shoot.

also Aero 308 top rail is proprietary to Aero Precision. if you use a DPMS upper and one of their handguards it is a different height.

[baih777]

this is what you want.

https://www.midwayusa.com/product/784553/dpms-upper-receiver-assembled-lr-308-a3-matte

the only to get the colors to match would be to cerakote the rifle.

[one918]

Thanks fellas for all the great info, and thanks baih777 for the pics with the TM lower. A lot of great info dreweckhardt! So in calling and emailing I've been told there is no one making gen2 style uppers (since I'm looking for the gen1). I really like the Juggernaught and SAA Dpms style upper. SAA said theirs is a gen 2, but then went on to say there is no difference other than roll pins in gen 1 and gen 2. This completely contradicts all the info I've been getting here as well as read elsewear.

To sum things up I found an AP M5 lower in stock and ordered it as I'd rather have a matched set. I'm still going to keep my TM lower and find the Dpms upper for a 6.5 build down the road ;-)

[baih777]

I can honestly say that the gen1 and gen2 are different. Bcg and threading for the barrel nut are different.

[one918]

I can honestly say that the gen1 and gen2 are different. Bcg and threading for the barrel nut are different.

Yours is a gen 1 upper correct?


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[baih777]

Yes mine is gen 1.

I actually compared it to gen 2.

[one918]

Yes mine is gen 1.



I actually compared it to gen 2.

Thanks much sir.


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[one918]

So quick update for those interested. I contacted Guntec and they make a
.308 billet upper. I was told when I called it's a Gen 1. That shall be my upper receiver to mate with my TM lower!


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