"A firearm should be considered a fighting weapon first. Any other use should be considered a bonus." -Me
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I am affiliated and work with SIONICS Weapon Systems in Tucson, AZ
I would stress again that I have 2 other MUR-1A's that fit all 3 Mega lowers properly. Maybe I should have sent the Mega lower as well so that they could see the issue firsthand, but I didn't know if the lower would require FFL transfers.
The only way to 100% avoid tolerance stacking issue is to buy a fitted upper and lower from the same factory. It would usually be advertised as fitted.
If it fits, even if it requires some force, but you can take the rear pin in and out at the field, I wouldn't worry about it.
What is tolerence stacking?
NRA Life Member Since 1993
Here is an example. S I want to machine my own AR-15 hammer, but I don't want to use a blueprint. So I ask a friend to mold his hammer part and send it to me. Then I take a pair of calipers and take the measurements, and do my own drawing. Then I get on a CNC machine and make the part.
Say the mold was only accurate to +-0.02" (Just example numbers here)
The calipers were calibrated and rated to be accurate to +- 0.01"
Then say my CNC machine will machine parts to 0.02" accuracy.
Then the dimensions of my finished part may be up to 0.05" off. Because the tolerances were stacked.
Theoretically that doesn't mean the part will be off by whatever the total sum comes out to be (in this case, 0.05), but it can be that much off.
I don't really know what the industry does to manufacture these receivers, but potentially something like this can happen:
Brand A and B are AR-15 receiver manufacturers. They each use a different point in the part to measure the dimensions. So Brand A has holes that tend to be off towards the forward part of the rifle, but brand B tends to go other way. Even though both manufacturers made both parts within milspec tolerances, the upper may be out of spec with the lower.
In reality I think most manufacturers generally machine things the same way; there may be differences in material, overall tolerances, quality control, etc. But an example like above where tolerances can stack isn't going to happen between most manufacturers.
My personal experience is the same. I've also not had many customers complaining about parts not fitting between different manufacturers. But I still may be wrong, so please take this bit with a grain of salt.