Shims?

No, & why would you want to make it a pain in the A when switching uppers or the lowers on different upper or a different lower.
Some have a bit of play, well good it's easier to pull the 2 pins & swap things around. Takes 30 seconds without having junk falling from the gun.
 
If it's the front pivot pin fitment, I have seen an o-ring used to tighten up the fit. You have to replace the oring if you separate the upper/lower but a 250 ct pack is 14 dollars from Amazon. If interested I will try and link/photo the brand.
 
' bedding a bolt action' is from my understanding, to solve problems with barrel harmonics. When assembled properly, the (newer) AR system is all 'free floating'. Some claim shimming helps but I've never seen any proof. Lots going on in the semi-auto action when the primer goes off so lower 'jitters' would be minor.
 
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The fitment from upper to lower has negligible affects to accuracy downrange compared to how your barrel and handguard fit together, or the use of a bipod on the handguard. Multiple folks shooting benchrest and f-class will affirm that 1) the size of the upper receiver at the barrel extension junction, and 2) the better the handguard/barrel attachments bring about more accuracy issues than how well the upper and lower fit together. Focus on good quality barrel, quality upper receiver, and you'll satisfy majority of your accuracy issues. The handguard diameter and how it mounts to the upper is going to also assist with this concern.

It's more annoying and less "custom" if they shift, but that's about it.
 
spotstalkshoot said:
If it's the front pivot pin fitment, I have seen an o-ring used to tighten up the fit. You have to replace the oring if you separate the upper/lower but a 250 ct pack is 14 dollars from Amazon. If interested I will try and link/photo the brand.
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Go to your local Menards or who ever. Get a # 41 O Ring at the plumbing dept.
 
Cant be good when a hammer in the lower strikes a firing pin in an upper causing shift. I use steel shims in the front and an accu wedge in the rear. When dry firing I can generally see about an 1" or so of crosshair shift across the bull.
 
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