Algodoo

Same model as my 1st delayed blowback gun, but using a different ammo type, and also thus needing a couple upgrades.
New bullets shoot 4 pieces in a line.

To operate, press R to close the Breach, then F to fire. The bolt will cycle back and push a new round against the Breach. Press F, then R, to finish rechambering.
To change ammo type, grab and 'shake' the grey block at the back of each bullet, to select the whole bullet, and delete as appropriate. Then C&P the other bullets into the magazine.

Bolt rarely doesn't cycle back; just pull it manually if this does happen.
Can be fired on full-auto just by using F, but it will be prone to malfunctioning. ~1/3 times it'll go through a complete magazine without a hitch.

Delayed blowback allows you to get more bang for your buck out of the same propellant spring, whilst also proving to be realistic, as in real life, the recoil mechanisms activate when the bullet's leaving the barrel; not when it's just been struck by the firing pin.