Hey bud, sounds like you're building a 300blk like mine. I also have the 300SPS for it though mine was built as a pistol. The 300blk was designed for a shorter barrel so the option of going with a carbine buffer tube or a rifle tube is up to you. If you go with a rifle tube and the rifle buffer/spring to go with it there will be tuning involved to get it to run right, but it will shoot smoothly with minimal recoil when tuned right. Seeing as it was designed with a pistol gas length barrel and carbine tube, going with that would be the easiest way to get it running right in the shortest amount of time. I'd also suggest an adjustable gas block when building a gun to suppress it. Feel free to reach out during the build process if you have anymore questions and I'll advise you as best as I can.