Synthetic ammo issues mac 10

[Dean]

Finally took out the mac 10 with Lage upper yesterday.  The firing rate was erratic and I got a lot of blown out casings.  I was using Federal 150 grain with the synthetic jacket.   Could this cause the problems or is it the upper itself?

[mike todd]

DEAN, next time you take that "mac" out be sure to have the original upper along. just in case. a quick switch back to the original upper and you would know if it was the ammo or the change. I try to have a good asst of my "proven" parts / mags/ ammo along anytime something new is tried. while the net can be a big help, there are just too many variables. its certainly better to use a "PROVEN" set or combination of parts .

[Born loser...in Montana]

19 hours ago, Dean said:

 Could this cause the problems or is it the upper itself?

Yes... all is possible. 

I take it  one thing  at a time. 

Ammo  first!

[Gaffshot]

Sounds like the ammo is reloads with too much powder.  Not many other causes for blown casings.

[mike todd]

GAFFSHOT, ammo is stated to be FEDERAL I would take that as a factory loads , many sub machine guns fire well b-4 a round is fully chambered.  known as "preignition" its a necessary part of any blowback action and the cases will show the effect. when things go wrong and fire even sooner, it will be much more obvious as in this case.

[Dean]

Called Lage and they guessed ammo because it was 150gr vs 115gr.  Plus the synthetic bullet itself.  Will try some round nose jacketed lead rounds.