I have put together a couple of AR10's . You did buy it as one assembly with the correct recoil spring and the correct buffer ? The rifle chamber may be a little large and you say you are wanting them to take it back . When you buy another one are you going to hold it in your hands first if so take your go , no-go gauges with you break it open pull the BCG and put the no-go in the chamber to see the fit . I buy the lower and upper then put the parts I want to go with for them , fire control group , stock and recoil spring and buffer . Then buy the barrel I want with the BCG install it in the upper after I check that the upper is bored true or lap it and the extension true to each other ( for accuracy ) properly torque the barrel nut and use a good clamp on gas block again for accuracy not a pined or set screw one . I also check that the buffer retaining pen isn't interfering with the BCG traveling back some of them are a little long and drag on the BCG .I have also had the bolt catch be a little long and drag on the BCG lifting it upward causing it to bind as it travels backward . If the buffer spring or buffer are too light it will open too soon causing the brass to not be supported in the chamber while it still has gas expanding in it . I know that you know all of this stuff already but some times we get frustrated and forget things . They may have put too many shims between the barrel and barrel extension also causing a looser chamber , another good reason to go with a clamp on gas block they line up easier so your gas tube is lined up with the barrel nut and you don't need shims to line up the nut and gas tube or get a barrel nut that allows the gas tube to be aligned with out shims . Good luck with what ever rout you decide to go with it depending on what the company is like to deal with . Dave