Mossberg 500 action opening

[buffalohunter]

I got my grandson a Mossberg 500C Bantam 2 barrel combination 20 gauge. Yesterday I sighted in the deer barrel. I rested the wooden forearm on a sand bag and upon firing normally, I noticed the action had fully opened, ejecting the fired hull. After repeating this several times I tried firing it holding the forearm with my hand. The action opened partially but did not eject the hull. I have fired the shotgun barrel with light skeet loads and never experienced the action opening. I was using Federal 3" with a Barnes sabot slug with the rifled deer barrel. The heavier recoil obviously opened the action, but is this normal ?
Also, with the rear Williams sight at its lowest setting it still grouped 3-4 inches high at 50 yards.
As a side experience, I had to file the shotgun barrel extractor slots on the shotgun barrel deeper to reliably extract fired hulls.
Frankly, since police and military use this action I expected better performance.

 

[Oldnamvet]

The action unlocks after the hammer drops so the greater recoil is giving you an "assist" in opening the action. I see this even with lighter target loads on my Winchester 1300 pump. If I don't touch the forearm when I fire, it will completely eject the fired hull so all I have to do is push it forward to get ready again. To me it just gives me a millisecond or so quicker time in chambering the next round.
Filing out the extractor slots?? Never have known anyone to have to do that. But then I only know about 10 people with 500's.

 

[SASS#23149]

I have an old well used 500 in 12 guage that does the same thing.
I"m surprisedd that a new one does it though.
There is prolly a fix for it but I"ve not seen the need to bother...so far.On a new gun I would be calling Mossberg I think.

file the extractor slots? that is a new one on me too.

 

[30-06 lover]

Sounds like the heave recoil of the deer load is causing the bolt to come back which in turn causes the gun to "self pump". Nothing to worry about. I have had several shottys do this with lighter loads. Be safe.
-Mike

 

[Fumbler]

Sounds like the heave recoil of the deer load is causing the bolt to come back which in turn causes the gun to "self pump". Nothing to worry about. I have had several shottys do this with lighter loads. Be safe.

That's correct.
When you shoot a gun it recoils backwards. Your shoulder slows it down, but since the bolt/forearm assembly is now unlocked then it keeps traveling.

My 500 does it with heavy loads when I don't hold the forearm.

 

[Srigs]

The action unlocks after you shoot it. Its normal so no issue.