Can a new lower mess up a previously very accurate load?

[Kaldor]

Can a new lower make a good load shoot bad? I don't see how

No but it can make it more difficult to achieve the same results. I have a couple lowers with Hi Speed NM triggers and PRS stocks on them that are much easier to shoot more accurately than a lower with a CTR or UBR stock with a very short fast HiperFire trigger. My 2 accuracy lowers are absolutely identical other than a PRS Gen2 vs Gen 3, and they shoot exactly identical.

Not knowing his exact setup, it's hard to determine what the cause might be. All things being equal, and the lower being the only change, they should shoot identical. Any deviation from that adds additional variables.

 

[Double Naught Spy]

What happens on an AR when you pull the trigger? The hammer in the lower pivots up and strikes the firing pin buried in the upper. If the upper is not as tight to the lower, or some other geometry has changed, it's quite possible the strike of the hammer will move the upper enough to affect accuracy.

While possible, nobody seems to notice is as the change, if any, it far too small to be noticed or sorted out from background noise of groups.

AR15 Upper and lower fit affect accuracy?

I read all the time from people that the slop between the upper and lower on an AR has no effect on accuracy and was also told this by the local AR builder. This just make no sense at all to me how this can be when you look at the difference it can make glass bedding a bolt action rifle. It...

www.thehighroad.org

How important is the fit between upper and lower and accuracy ? - The Firing Line Forums

How important is the fit between upper and lower and accuracy ? The Art of the Rifle: Semi-automatics

thefiringline.com

The military actually did a study on this, cited here...

The often quoted Army test about loose fitting upper to lower, and how it doesn't effect accuracy ? - AR15.COM

Firearm Discussion and Resources from AR-15, AK-47, Handguns and more! Buy, Sell, and Trade your Firearms and Gear.

www.ar15.com

As Kaldor and others indicate...

No but it can make it more difficult to achieve the same results.

This is likely where the difference in accuracy is occurring at the interface with the shooter and not between upper and lower.

People will ask if changing the trigger will change the accuracy and the short answer is that it will not change the accuracy of the rifle, but it may impact that of the shooter.

Smyth Busters: Is a Tight AR-15 an Accurate AR-15?

Will these guys ever run out of firearm myths to put through their Truth Tester? Not yet! Brownells Gun Techs Steve Ostrem and Caleb Savant are back to test ...

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[Charlie98]

Not knowing his exact setup, it's hard to determine what the cause might be. All things being equal, and the lower being the only change, they should shoot identical. Any deviation from that adds additional variables.

This is likely where the difference in accuracy is occurring at the interface with the shooter and not between upper and lower.

Well... I guess the OP and I have, or have had, the same problem. I'm not a scientist, I'm just a regular guy shooting AR's.

 

[Charlie98]

The more I think about this, the more I have to laugh...

In an endeavor like shooting and handloading... where we strive to keep tolerances, in some cases, down to a thousandth of an inch, shimming, bedding, torquing, turning metal off, supporting, wedging, locking into place... that the relatively loose tolerances between two different AR lowers against an AR upper DON'T have an effect on accuracy... shooter aside.

 

[rsrocket1]

Thanks everyone for your help and advice. I'm going to try the accuwedges and see what happens.
I love this rifle, by far my favorite to shoot. I like it so much and that's why I started buying stuff to "upgrade" it.

I've got a new scope and an mlok bipod ordered for it. Now I'm worried that adding those will affect it's accuracy!

Anyone have any opinions on that?

My guess is that it will affect barrel harmonics. By how much? It may be insignificant but if your accuracy node is very sharp, it might change it noticeably.

It might be a good idea to load 5-10 rounds down by 0.2g and up by 0.2g using your new lower changing nothing else to see if you were in the middle of the node or on the edge of a sharply falling node. In the worst case, at least you get to go out shooting some more.

 

[AJC1]

The more I think about this, the more I have to laugh...

In an endeavor like shooting and handloading... where we strive to keep tolerances, in some cases, down to a thousandth of an inch, shimming, bedding, torquing, turning metal off, supporting, wedging, locking into place... that the relatively loose tolerances between two different AR lowers against an AR upper DON'T have an effect on accuracy... shooter aside.

You nailed it, because pulling a rifle out of a stock and putting it back in the same stock with diffrent screw torque can and has changed the "tune" savages are well known for being grumpy about this. A rifle is a resonance tuned instrument, and changing anything that effects the resonance can change poi or overall accuracy. Hell just the way you hold the dam thing changes the system.

 

[Yankee1986]

Changing the stock on a bolt action rifle is absolutely NO comparision to swapping lowers on an AR. Apples to oranges.

One question for OP. What made you think a lower from PSA was better or an "upgrade" from the stock M&P?
I'm not knocking PSA but they aren't exactly known for high end lower receivers or triggers for that matter.

Personally, I would swap the trigger in the M&P lower with a CMC, Timney or Wilson Combat drop in unit or a Larue MBT and see if that made a difference.

The CMC is on sale at Primary Arms.

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[mkl]

System harmonics?

 

[EricBu]

I've got a few AR uppers in lots of calibers (maybe 15 or 20 at this point in time), and a lot of lowers (maybe a dozen or so), some with match triggers, or higher end stocks, some with very basic furniture and milspec parts, some registered as NFA devices....a very very wide variety. And I'll tell you, sometimes it matters, and sometimes it doesn't. Sometimes a given upper/lower combo will slightly shift POI, but group stays the same...sometimes group changes dramatically, and sometimes it's very nearly exactly the same all around. So really, I guess the advice I'd give you is either got back to the good set up, or work your loads up again with the new setup. Knowing that if you change stock, buffer assemblies, or other parts, you might have to dial it in again is part of the fun of a modular gun system like the AR.