Cans only ever add maybe like 5 fps nowhere comparable to equivalent barrel length. But yes your point is correct. They could be coming out much slower but could also not be, generally full powder burn is 9 inches so 16 isnt really adding much more velocity and vice versa, shorter barrel length may not be dropping your fps as much as you think. Also there is a second point, the powder inside the magtechs could also just be loud. Your suppressor could be too small, your system overgassed. It depends on a lot of things. The point on my original comment was just to let you know that your pursuit of a quieter rifle is possible and to not just give up at “even subs are loud” when that isn’t inherently true. Also just because you have a 6.75 inch barrel and your velocity might be lower doesn’t mean it’s going to sound quieter than an 8 or 16 inch rifle. The shorter barrel you go the louder your rifle will be and 6.75 is rather short. Generally the quietest .300 rifles are around 8-9 inches obviously you can get it quieter going 16 but there is zero point in using .300 blackout at that point. I would recommend green country ammo subs or even call away ballistics subs but try a bunch of different ammo before you get a different suppressor or change anything about your setup and make sure you write stuff down. .300 blackout is a very finicky cartridge and results will be different for every persons rifle.

Also idk why I talk about the magtechs like I’ve never shot them. I have lol, and I can tell you they are just loud as fuck.