So ive been reading more and more about GPS Defense cutting down their 308 rifles to 18. So my ques is: what other calibers can you cut the barrel down to 18 without considerable lose in accuracy or FPS. Also are they cutting down factory barrels or aftermarket? Also i found a website the other day that started at 24 and ended at 10 barrel length. It also showed the FPS table for every inch cut off. But i dont rem the caliber.