Okay so how about a semi-automatic sliding breechblock rifle?

>>32945916
you mean a semi auto falling-block?

Krag–Petersson

>>32947390
Yes semi auto sliding-block.

How is Krag-Petersson rifle related to this subject?

>>32945916
It would have trouble ejecting

>>32947605
Its the closes to a semi auto
>the Krag–Petersson (uniquely, a repeater), which use a pivoting rather than a sliding block.

Madsen LMG

So you want a semi auto where the bolt moves at a 90 degree angle from the recoil impulse? Did you even think about that before you hit "Post"?

>>32945916
gas-operated long stroke piston. Put the recoil spring on the piston.
So here's how it would have to work, the gas operates the piston which pushes down on the top of the breech block. It's the only way it would work and still be viable. I guess the closest thing to a bolt carrier would be a sort of two pronged fork thing attacked to the piston, and as the piston comes back, it cams the bolt carrier down. Might have to do it at an angle, I don't see a straight 90 degree falling block working in any sort of self-loading fashion like that. Also the bolt carrier would have to be open in the middle to allow a new cartridge in.
However. I'm going to quote Jurassic Park:
"Your scientists were so preoccupied with whether they could that they didn’t stop to think if they should."

>>32949667
... Ever heard of a gas piston?

>>32952584
How the fuck does your gas piston move perpendicular to the barrel? You want a rack and pinion gear system in your gun? Cause that's retarded.