Interesting issue with my CZ 457 varmint MTR

[tazman]

The MTR is pillar bedded, but not like you would normally think. It has two pillars that the action screws pass through but there is a third piece of metal at the very rear of the action that fits into a notch behind the rear action screw. This allows for extra support but also allows for a bridging effect if the rear support piece is slightly higher than the pillars.
I can't think of a way to properly test this as it all sits in the bottom of the action/barrel trough and can't be easily seen from the side.

The rear piece is shaped like a horseshoe and sits in a small cutout in the stock. The top of it fits into a notch in the bottom of the action behind the rear action screw.
Because of the pillar bedding, you can torque the screws to higher settings and still have things work. When I torqued the screws down at slightly over 30 inch pounds during testing, the bolt became harder to work. I believe this was due to the action springing slightly. This should not happen if the bedding is even and level at all points.
Relieving the pressure returned the action to normal smoothness.

My last trip to the range, the rifle was shooting less than optimal. When I got home, I thought about things for a while and came up with a plan.
I cleaned the rifle thoroughly. I hadn't done this for about 150 rounds so it probably needed it.
When I put the action back in the stock. I used lighter settings than I had before with this rifle. I decided to use the same settings I use with my wood stocked rifle.
The rifle shot well today. not perfect, but well. While at the range, I increased the torque on the front screw slightly and left the rear one alone.This improved groups some more.
Now my settings are 20 inch pounds on the rear screw and 24-25 on the front screw.
I also found by experimenting that I need to torque the front screw first. If I torque the rear one first and after torquing the front one, I check the rear, the setting is different showing a slight increase in torque. This also indicates a situation with all the pillars and support metal not being perfectly in line.
My problem is I need to leave the stock as it came from the factory to follow the rules of the discipline I want to participate in.

I seem to have found a happy medium where I have not distorted the receiver by over torquing and still getting good results with accuracy.
If someone can think of a way to check if all the support points are at the right heights I would appreciate your ideas.

[barrabruce]

Try a bit of candle black on the action first.
Or mark out blue.
Photo may help a thousand words.

[tazman]

I will try and get some photos posted after tomorrow. I have a competition to go to and I don't want to change anything on the rifle until after the match.

[OS OK]

Trying to visualize your description taz . . . from this manual... https://cz-usa.com/wp-content/upload.../cz_457_en.pdf

but I don't see the part your talking about? This below is a blowup of the parts from the above manual...

[Mk42gunner]

I'm guessing part number 70 is just a bit taller than it should be.

This is the part that makes it a hard fix:

My problem is I need to leave the stock as it came from the factory to follow the rules of the discipline I want to participate in.

I can think of several ways to adjust the bedding, but none are probably legal with your restrictions.

Robert

[tazman]

I am considering just removing it. The action would still be fully supported by the two pillars with the screws. This would remove the stress from an uneven bed.
Part number 70 cannot be seen without dismantling the rifle.

[uscra112]

If it's removable, take it out and reduce its' height by some amount. Who can say whether it was done by you or the factory?

[tazman]

If it's removable, take it out and reduce its' height by some amount. Who can say whether it was done by you or the factory?

Very true.

I will test the rifle with the piece out before actually modifying it and see if having it in there matters. I may find out that piece is causing my fliers.
I wonder if having it in there loose would cause issues from just sitting in the hole with no pressure on it? Kind of like having a marble loose in a hole under the butt-pad.

[uscra112]

Make a dummy that's a bit short. Add shims under it until it just contacts the tang when the main screws are correctly tightened. Measure. Lap off the bottom of the real part to match.

Another way might be to use plastigage instead of the shims.

As for fliers, I dunno. But stressing the receiver as you described cannot be helping.

[Forrest r]

I've always ran my pillar bedded rimfires in the +/-45 inch # range.

A pea sized piece of playdough under the receiver near the pillar will tell you everything you need to know.

[tazman]

I've always ran my pillar bedded rimfires in the +/-45 inch # range.

A pea sized piece of playdough under the receiver near the pillar will tell you everything you need to know.

I didn't know I could take it that high. Now I have more testing to do.

[tazman]

After I got home from the competition today, I took the rifle apart and studied things a bit.
Here are some relevant pictures.


The small block of steel in the first picture sits in the stock in the crosswise slot at the rear of the receiver channel. The top fits into the slot in the bottom of the receiver at the rear shown in the third picture.

The problem turned out to be the metal piece was slightly too tall. I know this because the rear action screw is visible through the hole it screws into when the bolt is removed from the rifle. With the metal piece removed the screw fit about one thread further into the receiver than it did with the piece in place. With the piece removed, the screw fits flush with the inside of the receiver and does not interfere with the bolt whatsoever.
Apparently, the receiver wasn't sitting solidly on the rear pillar with the metal piece in place. When I tightened the rear action screw down enough, the receiver would flex a bit and begin to bind the bolt slightly. Enough so you could feel it when working the bolt.
Not certain what purpose it really serves except to complicate the bedding. With the metal piece removed the receiver fit nicely and solidly onto the pillars with no rocking or sliding around.
I am going to work with the rifle without the metal piece for a bit and see how it shoots. I can always put it back if things refuse to work. I can also remove some metal from the height of the metal piece if I need to put it back.

[uscra112]

Beats me what purpose that part could have. Recoil shoulder? For a .22???

Does CZ build more powerful rifles on that action?

Think of the screw as a micrometer, do a bit of simple trigonometry, and you'll know how much to take off it that block.

[OS OK]

After I got home from the competition today, I took the rifle apart and studied things a bit.
Here are some relevant pictures.


The small block of steel in the first picture sits in the stock in the crosswise slot at the rear of the receiver channel. The top fits into the slot in the bottom of the receiver at the rear shown in the third picture.

The problem turned out to be the metal piece was slightly too tall. I know this because the rear action screw is visible through the hole it screws into when the bolt is removed from the rifle. With the metal piece removed the screw fit about one thread further into the receiver than it did with the piece in place. With the piece removed, the screw fits flush with the inside of the receiver and does not interfere with the bolt whatsoever.
Apparently, the receiver wasn't sitting solidly on the rear pillar with the metal piece in place. When I tightened the rear action screw down enough, the receiver would flex a bit and begin to bind the bolt slightly. Enough so you could feel it when working the bolt.
Not certain what purpose it really serves except to complicate the bedding. With the metal piece removed the receiver fit nicely and solidly onto the pillars with no rocking or sliding around.
I am going to work with the rifle without the metal piece for a bit and see how it shoots. I can always put it back if things refuse to work. I can also remove some metal from the height of the metal piece if I need to put it back.

Did you compete today without that bedding block in the receiver/stock? You shot good today, right?

[OS OK]

Beats me what purpose that part could have. Recoil shoulder? For a .22???

Does CZ build more powerful rifles on that action?

Think of the screw as a micrometer, do a bit of simple trigonometry, and you'll know how much to take off it that block.

I think that receiver will work with a 17 HMR & 22 WMR with a barrel change. https://www.czub.cz/en/firearms-and-...cz-457-miniset

[tazman]

I think that receiver will work with a 17 HMR & 22 WMR with a barrel change. https://www.czub.cz/en/firearms-and-...cz-457-miniset

That is correct.

Did you compete today without that bedding block in the receiver/stock? You shot good today, right?

The block was in there. I had changed the torque on the screws to keep the receiver from flexing. Still not very good. Too many fliers. Just better than I had been doing recently.
I am looking forward to trying it without that block in there. The bedding seems much more solid without it in there.

[OS OK]

How did it group with that bedding block left out?

[tazman]

How did it group with that bedding block left out?

I haven't fired it with the block out yet. I will find out next trip to the range.
I have high hopes for it.
The good score I shot during the competition was with a different rifle. CZ 455 Varmint Evolution.

[Forrest r]

https://youtu.be/iSIUtraui00

Just something to think about

[tazman]

https://youtu.be/iSIUtraui00

Just something to think about

I have never considered torque settings that high. Not to say it won't work, I just hadn't considered it.
The CZ company video suggested around 30 inch pounds so that is where I was working. There and a bit lower. That may well change without the small metal piece in there.
Turns out, they call that small metal piece a recoil lug. Can't imagine the need for it on a 22LR. Seems like another possible failure point to me.
If I get the chance, I will do some testing on torque settings without the lug in there very soon.