So, hand feeding a slug into the breech and than further push it with the pin probe is no problem.
I do notice ‘bite marks’ on the slug when I push them back out. Showing that the do hit something on the way into the barrel. (Only when fed from the magazine with the pin probe. When I use the pellet probe there is no problem)
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Although I have a Maverick 30 cal sniper shooting slugs with a huma pin probe, rather than an Impact, hear me out. I had the exact same issue. On the Maverick, the cheek rest needed to have material sanded/filed from it because it was keeping the magazine from seating fully into its "home" position, which means the slugs were off center when being pushed into the breech. The mag FELT like it was fully in it's home position before the sanding, but it simply was not all the way home. In the case of the Maverick, I could verify that the cheek rest was the problem by simply removing the cheek rest and then inserting a mag and firing. With the cheek rest removed there was no feeding problem.
I'm not familiar with the Impact, so I don't know what could be interfering with the magazine fully seating to it's home position. I hope it's something that is easily solved. Also, since you have already been forcing the pin probe really hard (like I did -- I was pissed), the pin probe might be bent now -- I was amazed that my pin probe didn't get bent.
[EDIT -- definitely do the following -- you'll be glad you did.]
One other thing I did, that I maybe didn't need to do (but I'm still glad I did), was to deburr the breech transfer port hole. I did it by using some bamboo shishkabob (spelling?) skewers and a hand-held drill. I took the barrel off, of course. I put one skewer into the drill like a drill bit and inserted the skewer into the transfer port hole of the barrel and worked it like I was trying to enlarge the openings of the hole. The bamboo is soft and gets chewed away into dust as it deburrs the edges of the hole. It's a very safe way to deburr the hole. The ends of the skewers will break off in the breech. Keep the breech end down to keep the bamboo dust and busted skewer ends from falling toward the muzzle end. You will need to clean the dust out of the breech and you should take the breech o-ring out and clean that groove and clean the o-ring and then put it back if it's still good, else replace the o-ring with a new one. I cleaned the bamboo dust out with q-tips dipped in rubbing alcohol -- and used dry q-tips as well.
[another EDIT -- just to be a bit more specific about the deburring process]
Use a variable-speed drill so you can go slow with the bamboo skewers. You only need to polish off the microscopic saw-like edges. You don't want to bevel the edges -- just deburr them. Before you start, take a cotton q-tip and run it around the edges of the transfer port hole to see if you can get any cotton fibers to be snagged off of the q-tip. If cotton fibers are being snagged, then you have some deburring to do. If no cotton fibers can be snagged, then there isn't any deburring to do.
stovepipe
