Quality Differences Between M&P15 Lowers and Others

McE · Oct 13, 2015

I'm starting down the SBR road and was wondering if there was any meaningful difference between the lowers across the M&P line.
Am I wasting any potential by SBRing a simple Sport lower? Is its inexpensive price tag reflected in something in the manufacturing process? Am I limiting myself in any way?
All I can tell is the fixed trigger guard but that's not a big deal. I have a few different lowers to choose from for the SBR, but I feel the M&P15 Sport is the best one I've got.
Or should I buy a new stripped lower to get my money's worth, as if there is some discernable difference in quality or parts compatibility of lowers?

scooter123 · Oct 13, 2015

From a functional standpoint the only real difference would be in the trigger and the butt stock. If you like the butt stock and trigger in your 15 sport I don't see any reason at all not to use it. However, if you want to improve the trigger and try a different butt stock I would suggest that you build a new lower based on a stripped lower.

Note, I would expect that you will have to look into a different recoil buffer for your SBR Upper. Because the gas system will probably be vastly different than with your 15 Sport Upper. As a result you may have to experiment with different buffers to insure reliable function without "over gassing" the receiver.

One alternative to doing this experimentation with various weight buffers is to build your SBR Upper with an Adjustable Gas Block. One I like enough to use in every one of my builds is the Syrac Ordinance detented Adjustable Gas Block.

crracer_712 · Oct 13, 2015

I built a lower with an M&P 15 lower I bought from Buds, not a sport lower. The finish on the S&W lower is excellent compared to a lot of other, but then again, it was much pricier than some of the other stripped lowers as well.

Tbat · Oct 15, 2015

For the most part, standard lowers are pretty much the same from reputable manufacturers. It all depends on what roll mark you want to rock, what finish you get, and if you want the ability to fiddle with the trigger guard.

kwikvette · Oct 16, 2015

If you already own an M&P lower, keep it and use that money for upgrades such as a different trigger and/or grip and buttstock.

A new trigger will make a huge difference.

McE · Oct 16, 2015

Can't make sense of difference between forged and billet lowers.
Still have lots of reading to do.

hanken · Oct 16, 2015

A forged lower is a casting and a billit lower is machined out of a solid piece of aluminum.

scooter123 · Oct 16, 2015

hanken said:
A forged lower is a casting and a billit lower is machined out of a solid piece of aluminum.

This statement is incorrect. Forging is a process where a metal is pressed into a form at high pressure. If the blank is pre-heated it's called a Hot Forged part. If the blank is cold going into the die, common with many grades of aluminum, the resulting part is simply identified as Forged.

One major advantage for forging is that the Grain Flow in the material can be designed into the process and with the Grains in the metal flowing in the correct direction or pattern the strength of the resulting part is enhanced. Another positive is that with most alloys used for Forging the Work Hardening produced by the Forging Process increases the strength of the finished part. Finally, if the material and process design is down well a Forged Part can actually prove MORE durable than a part machined from a solid Billet. As for why, when you machine a Billet you cut across the grains in the metal, with Forging the grain can be made to follow the various contours.

Quality Differences Between M&P15 Lowers and Others

McE

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scooter123

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crracer_712

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Tbat

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kwikvette

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McE

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hanken

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