POF has their P415 piston system and you can get just the 10.5" upper and use whatever lower you'd like although their lower is one of the best made. I have a 12.5 piston P415 in .223 and they're bomb proof. Can get the uppers from places like
https://www.cflgunandtactical.com/ and others. It has a wider range of gas adjustments than my Sig piston and you can even turn the gas off completely. The POF is like hitting the easy button. Super accurate, soft-shooting and a tank. I really enjoy shooting it. It'll never be as light as a DI gun though.
My 300 black is a Sig Spear LT which, IMO, is a bit overpriced and the barrel's fast twist rate dictates you run just subs (but not plated like Berry's) or if you need supers, they need to be monolithic bullets if you're using a suppressor.
The Sig, with no buffer tube, is tiny when folded. If you're using the Sig stock or Kate Moss stock, when folded, it doesn't increase in thickness much and stays compact. Something like the Law Tactical folding stock adapter makes for a pretty thick package when folded and adds substantially to the length of pull. The adapter which slides into the back of the BCG is also heavy so there may be some tuning necessary. I ran a titanium BCG and any gains were negated by running the Law folder.
You can get just the Sig upper too but again, it's overpriced IMO. It'll work on any mil-spec lower with the Sig adapter. It eats everything I throw at it including Hornady's 190gr. Sub-X rounds. I've had issues with those bullets in other guns. The piston system is also very robust, and like the POF is duty capable.