I have been using the 0.250" slugs, and have never tried the 0.249" version. The smaller ones might be even better, but its best to try both sizes in your specific liner. FX liners runs tight a lot but there are more than a few loose ones out there in rifles.
Oh waoooo i got super frustrated when my impact m4 wasn't hole on hole at 90 yards, i thought the i should be able to do that and i started analyzing how and why the gun sometimes opened up and shoot from half inch group now and not the next and backwards up to 3 or 4 inches having slow spread in fps ,and I stiffer up the barrel liner with cf sleeve then put groove on the cf sleeve for an o ring and then carefully thread in the whole barrel sleeve until passing the threads all of this making sure every o rings passes through good and then after 4 o rings install is much much better i don't have half inch consistently yet but I am trying and will keep trying and upgrading my gun until i get half moa or somebody posted the RTI mora saying that is a moa gun out the box if bought from talon tunes with altaros slugs if with in a year i can't get my m4 at least moa consistently 100%, i will try another brand until then I hope i can achieve my goals because i love everything about this platform....
