My lead stock insert has been backordered since Custer was a Corporal, so I've been using home made weights that I made by inserting lead bullets into balloons for my buttstock. When I shoot out my current barrel a front lead sleeve will get put under the NM freefloat tube (the barrel sleeve weight is never backordered). Eventually I'll have to figure a way to fill the pistol grip with lead too.
I may not be a good enough shooter to see the difference between a few ounces on the target, but once you add a couple pounds of lead it gets a lot easier to keep the rifle on target (once you get their with NPA).
Machine guns are awesome until you have to carry one.