Where are you getting extra screws when on straps out?
As others pointed out, Midway and Brownells, ( and Amazon ) offer firearm screw assortments.
The vast majority of screws, used on firearms, are offered in these assortments.
There are screws with thread ~ pitch combinations used that are in the normal or non-firearm category on firearms.
To buy either in multiples, ( usually at a much better price, for example, instead of .75 cents how about .12 ~ .05 cents each ) and have a much better choice of head style, internal wrenching, pan, slot, etc ,,
"Correctly Identify" the thread you want, inch ~ metric being the majority but there's a few more.
This can be a challenge with some "Odd Ball" applications, not just guns.
Most thread charts only show the most popular thread size ~ pitches in common use so if you don't have a chart that is complete, ( listing all pitches ) it can hinder identifying a fastener in question.
Side note, if you may need a tap to chase a thread, source it at the same time. For blind holes you may want a Bottoming tap and one can be made by grinding off the tip of a Starting tap, NOT Red HOT grinding either.
After IDing the fastener search Ebay for the style & pitch and I've also searched Amazon for fasteners and found it to be a Major PITA because of the way their search engine works.
You can also find "Industrial Supply Houses ~ Businesses" in your area and ask them and then there's Mcmaster-Carr along with a large variety of fastener suppliers on the web.
I did the same thing with gas block roll pins, detents and springs etc, etc.
If you ever need anymore, this is a good source for most of the small stuff.
AR15Xtreme is a top quality manufacturer of AR-15 rifle parts, located in Los Angeles, CA. Our goal is to offer precision-made high quality parts using the best materials at an affordable price. Established in 2013, AR15Xtreme has quickly become a trusted name in online rifle parts.
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