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I have heard some horror stories of issue weapons.
The thing that sticks in my head is that I was going to use this upper to build up my own rifle, and I am worried that the slop may have a negative impact on accuracy and wear on the gun(not that I expect it to be a heavy use gun).
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upper, lower "slop" would have no effect on accuracy, as the upper, barrel, bolt, and sights will all move together.
this is all to common in the ar15 / ar10 world, as long as they pin together there is nothing to worry about.
i have had to swap uppers and lowers to create the right fit, i have had uppers that wouldn't fit some lowers (various manufactures).
think about it for just a second one tool may cut 1,000 parts, this tool gets a little more dull each time it cuts. part 1 and part 2
maybe to close to measure, but measure part 1 and part 1,000 and you will see the results of this tool getting dull. the problem is
no one knows exactly when the tool will become dull enough that it needs to be changed until there is a problem. in the end you
may end up with a lower that was milled with a new tool, and an upper that has been milled with a tool that is ready to be replaced.
for those anal about this "slop" there are ways to mask this, such as o rings, accuwedge, set screws, ect. no manufacture is immune from
this issue as it is just the way it is.
for hundreds of dollars more some people choose mega machine for the matched sets. while mega matched sets are usually tighter fitting
it isn't magic, or better workmanship they just employee someone to physically test fit uppers and lowers until they get the desired fit.