Need some input trying to identify a feeding issue.

[CZ Shooter]

The shotgun in question is an HQ 105 mag fed bullpup.

It is still somewhat in the break in period, but for some reason it does not like low recoil Score brand slugs.
Being that I'm one of those people whom can't just accept it without trying to get to the bottom of it first, I would like some ideas as far as narrowing this issue down to something that makes sense.

When trying to chamber the first round from an open bolt (or closed bolt by racking it in), the first round likes to jam against the top of the chamber at a steep angle.
The way to solve this is either by nudging the bolt back just a hair and then releasing it, or taking out the mag and shaking out the shell.

First I thought it was a mag issue, but I quickly abandoned this idea by trying different mags with same results.
But here is the kicker...it chambers the Score brand high velocity slugs just fine with 100% reliability.

So I put them side by side and tried to find any physical differences.
I noticed that the high velocity slug is ever so slightly longer (by about 1 milimeter) and it weighs in at 1 1/8 Oz vs the low recoil one only being 1 Oz.
Otherwise the construction, height of the brass and the material used are exactly identical.

So I figured that maybe the extra mm of lenght was making the difference (allthough if anything it should be the longer shell jamming up, not the shorter one)?

So I run a batch of Remington sluggers, that were identical in lenght and weight to the low velocity Score slug, except they had higher velocity and the brass was higher.
The Sluggers cycled flawlessly.

That blew the slightly different lenght or weight theory as the reason for the jam right out the window.

What em I missing here?

 

[EL34/6L6]

Can you only chamber a round by manually racking the bolt? Or is there a bolt release button?

 

[CZ Shooter]

Can you only chamber a round by manually racking the bolt? Or is there a bolt release button?

Yes I can do both.
If inserting mag on open chamber, there is an AR style paddle release that I can press, to send the shell into battery.
If the bolt is closed, I can insert the mag and rack one in manually by using the bolt handle.
Except with these Score low recoil slugs, the rate of failure to feed is around 80% and I can't figure out why.

Even after I get the first one in, there is about 80% chance that there will be another exact same type of F2F malfunction by the time the 5 round mag is empty.

 

[Butcherbill]

With an empty mag inserted and the bolt locked back, have a look at how smooth of a transition it is from the front edge of the follower to the bottom lip of the feed ramp. As well from the feed ramp to the lower edge of the chamber. Had an issue with a mag fed pump feeding shells, the transition from the feed ramp into the mouth of the chamber wasn’t very smooth. Shells were hanging up on the bottom edge of the barrel as there was a small wall, I blended it with a dremel and mirror polished it and it feeds 2.75” shells 100% of the time.

Still has a problem with 3” shells but they hang up on the front edge of the mag, I dimpled the front of the mag follower so the shells sit at a higher angle and that helps them clear the front edge of the mag but it the mag isn’t long enough front to back to reliably feed a 3” shell, which isn’t an issue as I rarely shoot them.

 

[CZ Shooter]

With an empty mag inserted and the bolt locked back, have a look at how smooth of a transition it is from the front edge of the follower to the bottom lip of the feed ramp. As well from the feed ramp to the lower edge of the chamber. Had an issue with a mag fed pump feeding shells, the transition from the feed ramp into the mouth of the chamber wasn’t very smooth. Shells were hanging up on the bottom edge of the barrel as there was a small wall, I blended it with a dremel and mirror polished it and it feeds 2.75” shells 100% of the time.

Still has a problem with 3” shells but they hang up on the front edge of the mag, I dimpled the front of the mag follower so the shells sit at a higher angle and that helps them clear the front edge of the mag but it the mag isn’t long enough front to back to reliably feed a 3” shell, which isn’t an issue as I rarely shoot them.

There is really nothing I that could be called a "feed ramp" on this one, but the edge of the shell does get caught up at the upper portion of the barrel wall (or the little step that transitions from the reciever to the barrel).
I can polish it up a bit and see what happens there.
Thank you for this idea.

 

[Butcherbill]

There is really nothing I that could be called a "feed ramp" on this one, but the edge of the shell does get caught up at the upper portion of the barrel wall (or the little step that transitions from the reciever to the barrel).
I can polish it up a bit and see what happens there.
Thank you for this idea.

That was the biggest issue I had, once I took the dremel to it and blended it to reduce the step it made a massive difference. Dimpling the mag follower upwards helped to push the nose of the shell up and at a better angle into the mouth of the chamber, which will only work if the mag follower is metal.

Give your shells a measure, I’ve always noticed unfired shell length is all over the place. The shorter the shell the better it fed in my gun, which would explain my problems with 3” shells.

 

[CZ Shooter]

That was the biggest issue I had, once I took the dremel to it and blended it to reduce the step it made a massive difference. Dimpling the mag follower upwards helped to push the nose of the shell up and at a better angle into the mouth of the chamber, which will only work if the mag follower is metal.

Give your shells a measure, I’ve always noticed unfired shell length is all over the place. The shorter the shell the better it fed in my gun, which would explain my problems with 3” shells.

Thos is why I'm so puzzled as to why it will feed the slightly longer shells without a hickup...it should be the other way around.
Maybe the way they are balanced (weight wise) is slightly different.
Will try to push them off a table while keeping a ruler behind them to see wich one falls off the edge first, as the fulcrum point could be the difference between the feed and a jam.

 

[Butcherbill]

Well good luck, from what I’ve experienced in anything I’ve had feeding issues it either been mag or feed ramp/chamber mouth related but it could just be the ammo. Not uncommon for shotguns to not run well with cheap low brass shells.

 

[CZ Shooter]

Well good luck, from what I’ve experienced in anything I’ve had feeding issues it either been mag or feed ramp/chamber mouth related but it could just be the ammo. Not uncommon for shotguns to not run well with cheap low brass shells.

I had put any shell I could find on a digital scale last night.
The Score low recoil shell is on average 3 grams less than all of my other stuff.

My theory now is, that it gets thrown upwards a bit higher than the other shells due to the lower overall weight, which could be at yhe very least a contributing factor.

Also, I had mistakenly thought that it gets hanged up just before the chamber, but I realized that the top edge of the shell is actually already inside the chamber (my up close vision is not the best anymore)...but the angle is so steep, it is unable to continue into battery (pretty much 45 degrees)

 

[Butcherbill]

Yeah could very well be, another thing I had to do initially when I got mine was to trim the mag spring shorter. The gun came with one factory mag which fed reliably, I bought two extra aftermarket mags that wouldn’t reliably feed shells. I took them apart and compared spring lengths and the factory mag spring was shorter, I trimmed the longer ones to match and the functioned like the factory mag did. Mine did not like the heavier spring tension for some reason.

If you have an extra one to sacrifice might be a worthwhile experiment, just a thought though. Sounds like you’re on your way to sorting it out.