Snakeman721 wrote:Kinda off topic....what is "tolerance stacking"? Sorry, I never heard the term.
For any part, there is a minimum and maximum tolerance. Let's say the ideal diameter of a pin is .154", which is the size of a mil-spec trigger pin on an AR15. For a top tier rifle manufacturer, the trigger pin hole must be .154". For other companies, their tolerances may allow them to have .154"-.156" pin holes.
Conversely, say a company has a hypothetical tolerance that allows a +/-.050" variance in size. If you have a 0.00 on both, it will be a perfect fit. Conversely, if you have a +.050" on one and -.050" with the other half, you likely will have a solid fit. However, if any of the tolerances are off from that even split, you have fit problems relating to tolerance stacking. A loose tolerance on both upper and lower receiver is what causes a rifle to rattle.
There are products on the market that exist because of tolerance stacking. The "Accu-Wedge" was created for loose tolerances between upper and lower receivers. Anti-walk pins were created for rifles with loose tolerances on the trigger and hammer pin holes. Oversized pins were also created for loose tolerance trigger and hammer pin holes.
When you have loose tolerances, it can affect accuracy, or it can cause the rifle to wear abnormally and you will see premature component damage/failure.
On the flip side, too tight of tolerances can cause difficulty in disassembly and maintenance. It can also increase friction and also cause excess premature wear.
Tolerance stacking is very common in lower and middle tier manufacturers. I have had to hammer the take-down pins out of several rifles when new. I've also held some that were so loose they sounded like maracas when you shook them.