Here’s a picture of my Dad shooting my 450BM pistol. The gun has an AR side charge upper with the BCG cut in half so it will operate inside the upper without going into the buffer tube (which isn’t there). There is no spring, etc. and the gun is manually operated (Google Solo 300 for more info).
I was “having fun with science” and started with a 16” barrel and cut it down 1” at a time to check velocity changes.
I can’t remember where the results are, but it was taking so long to chop the barrel each time we didn’t complete the test. I think the picture is of it in its shortest length, of 4.5”. I had the barrel threaded, but my Silencerco Hybrid was no match for the blast, leaving it still VERY loud. The gun is now a SBR and wears a 9” barrel from McGowan.
Enjoy the ring of fire in broad daylight.