Huben K1 .25 + Altaros slugs =?

I Put a caliper on one of these Altaros smooth slugs..... .254 at the widest point. AS mentioned before , this ammo bulges in the middle and tapers off at the back significantly.The magazine.... I see now rotates fine till it gets to the fire position 12: o'clock and locks up each time....I haven't taken one an apart so far ( and may not get into that )....so I'm not sure what it's catching on in that position. Would like to find out , and maybe solve what's wrong here ,cause I like this ammo for what I'm looking to accomplish eventually.
 
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