dail adjustable spring blow-back/delayed roller guns?

[Big Chudungus]

Typically motorcycles got a adjustable rear shock spring for diff weights, which is more just height really, but you get the point.https://www.youtube.com/watch?v=2pOCnVqNqJo

I hear a great feature of FAL battlerifle VS HK G3 is the FAL has adjustable gas to easily match it to wide range of ammo.

Seems like it would be easy to (somehow) make a gun's spring adjustable on the fly (without swapping springs).

Also, why has pneumatic shock absorbers never used in guns AFAIK. I'm thinking instead of typical automotive sealed unit, for a gun it would be loose dry, non-sealed piston, without any rings or 0-rings, just tight enough fit to put up resistance in the milli-seconds of a bolt blowing back, and maybe retain a bit of pressure to push the bolt back at lower energy. Maybe in conjunction with spring or even gas, maybe not. Air is light, free, provides cooling and never "wears out".

[divingin]

Combustion residue or dust and dirt would likely screw with tuning. I doubt that you could get that close a fit without making the manufacturing/selection process rather expensive. Air doesn't wear out, but anything that holds air in a mechanically variable assembly does.

Fixed springs are cheap and easy to maintain and replace.

I do suspect that larger arms may well use what you're talking about though: mid-caliber cannons, artillery and the like may have recoil dampening techniques that we don't see on consumer firearms. Most military stuff tends to ignore 2 things that are more important for civilians though - initial cost and maintenance expenses.