problem with my 1911 sig tacops

[cdin01]

so heres the deal, i have owned this gun for a little more than 2yrs. never had a problem until recently and cant figure this one out. when load one at a time through the ejection port the gun runs fine, given its 1 shot at a rip. i run the same loads through a mag and the slide doesnt fully close. thought it was bad loads so i loaded the "bad ammo" 1 at a time and boom no problems. ive tried new mags, the feed ramp is polished, happens with only lead loads, any ideas?

[Larry Gibson]

What are the cast bullets sized to?

Have you taper crimped the case mouth flair so the case mouth is closed but not roll crimped?

When single loading are you letting the slide slam shut on it's own chambering the cartridge?

Larry Gibson

[Mk42gunner]

Which cast bullet and what is the overall length?

Robert

[MtGun44]

Increase your taper crimp and/or seat boolit slightly deeper.

Bill

[ultramag]

Increase your taper crimp and/or seat boolit slightly deeper.

Bill

This.

I would lean towards seating depth and/or boolit size based on my personal learning curve with loading cast in semi autos.

[9.3X62AL]

All the above, plus use old-school Colt 7 round magazines--or mags that duplicate Browning OEM characteristics. The farther a magazine departs from Browning's original design, the more likely it is to create problems with functioning.

[knifemaker]

When you reload your ammo, you need to remove the barrel and do the ka-plunk test of the ammo. Size the ammo, seating deph and a .470 taper crimp at the mouth. test for proper sizing by dropping the round into the chamber mouth and see if it seats to full dept.
Based on your statement, your rounds are just out of specs on either being too long on the bullet seating or not enough case mouth taper crimp. Reason I say this is your statement that when single loading they seat and fed from mag they do not.
When you single load with no loaded magazine in the gun, your slide is not slowed down by friction from sliding over a cartidge in the chamber. This higher slide velocity allows the round to seat into battery.
With the mag in place and feeding from the mag, your slide is slowed down enough it will not fully seat the out of spec cartidge. I am willing to bet you have the cast lead bullet seated to far out and it is hitting the barrel lands.