25 February 2019
There is lots you can do to an AR-15 or variant to make it run at lower pressures. Be aware that modifying the weapon then using any full pressure loadings without returning the weapon to original condition will cause undue wear / damage to the firearm and probably whomever is shooting it.
1. You can change out buffers. Look on google or whatever and check buffer weights. They are all different weights for one reason or another, some even use them to change the cyclic rate of fully automatic weapons. Most buffers are very reasonably priced and you can either use different self contained (standard) buffers or you can use something like the JP captured spring and buffer assembly. If you do use the JP, it can be a bit of a bear to take it out of the buffer tube till you develop a technique.
2. You can change out recoil springs. Again, these are readily available, fairly reasonably priced and easy to change. You can buy them in different lengths, weights, etc.
3. You can trim 3 coils off your buffer spring, then as you shoot a load, trim off 1/2 a coil at a time till it functions. If you feel a dull thud on your shoulder during recoil, stop. You cut too much and will have to buy a new spring.
4. You can lighten your bolt carrier. I said bolt carrier, not the bolt. If your rifle has a "full auto" style bolt carrier in it, ditch it for a light weight semi auto only carrier. If you already have a semi auto only carrier you can trim a little at a time to help the rifle run. With light loads, you probably cant trim too much off. I don't know that for a fact, I have never gone that far to know. All the carrier does is add mass to the bolt, cycle the gas to unlock and give it an axis to turn on. Trimming the carrier is best done on a mill for precision.