For Christmas I was given a new Sig Collapsible stock with an AR15 adapter kit to run it on a standard AR15. The stock won't fit around the buffer tube mount on a standard AR15 lower. I saw one guy filed down the lower and was able to reteo fit the stock, anyone else have experience with this. Or I bite the bullet and get a Spear LT lower. I'm running a Sig Spear LT upper