There are firearms that are going to need a lot of attention when it comes to changing parts or doing upgrades on  them.   I think it’s a good idea to drop off the gun and the parts with a gun smith unless you know for sure that the part you have is a “drop in”.    From my experience, most gun parts and accessories will need some fitting.   This is where people tend to break rifle stocks or rails because they start forcing things when they really should be filing something.   I know several gun shops that get tired of people calling them up and claiming that a firegrip or rifle stock was broken, when the gunsmith knows how it really happened.    I did several AK stock changes and every one of them was different.   I even found areas that need to be filed down where on similiar guns, it just snapped right in.

Gun barrels on Glocks are almost as easy to swap in and out as a Glock magazine.   I have seen plenty of worn out guns before, but if you realize that each part wears out different over time, be prepared to do maintenance and know that at a certain point, it’s time to replace it, before it break on you.   Sig Sauer and Glock barrels are drop ins.   If you think  you’re seeing a decline in accuracy and can’t see the grooves in the barrel.  It’s defanitely time to swap the parts out.    The Remington 870 is one of the most versatile shotguns in existance.   Just by unscrewing the barrel and replacing it with another,  you can turn a duck hunting shotgun into a home defense weapon in seconds.