What’s going on with this Mk10 bolt?

20 March 2024, 12:03

I recently acquired this Mk10 bolt. It has an unusual length cocking piece, and a corresponding alteration to the cocking cam on the bolt resulting in the firing pin moving back less. Works perfectly in an FN action with Sako trigger, but doesn’t feel right in a 1937 Portuguese with military trigger - notchy on the uplift in the final stages.

Does any one know why this was done (some form of speed lock?) and what it was called? I bought it mostly as it had the right bolt face

20 March 2024, 16:28

Bobster

Try swapping a military cocking piece.

20 March 2024, 19:53

J Wisner

Yes that is a early speed lock change they did in the later production Interarms Mark X bolts
You have to keep that cocking piece with that bolt so the cams match up
Have seen some rough cuts on those, but the cams can be honed/lapped in for smoother operation

Have one of those actions in the shop now, just installed a M70 safety to it and am barreling it to a 458 Win Mag for the customer

So very close to the current Zastava M70 bolt and cocking pieces

Also the FN cocking piece you show appears to be from a JC Higgins M50, as it has the angled sear face

Jim Wisner

20 March 2024, 20:46

Duane Wiebe (CG&R)

Have personally seen two of those "speedlock" cocking pieces fail (brake)

Either outrageously hardened or perhaps a casting..the nose is just hanging out there a bit far

20 March 2024, 22:11

J Wisner

They are harder than Wood Pecker lips.

I have to use a carbide end mill to cut them when installing a M70 safety and even then its a hard job.

Jim Wisner

21 March 2024, 00:48

brnomauser

quote:
Originally posted by Bobster:
Try swapping a military cocking piece.

This doesn’t work - see the photo of the cocking pieces, unlike FN and standard military they’re incompatible with this speed lock set up

quote:
Originally posted by J Wisner:
Yes that is a early speed lock change they did in the later production Interarms Mark X bolts
You have to keep that cocking piece with that bolt so the cams match up
Have seen some rough cuts on those, but the cams can be honed/lapped in for smoother operation

Have one of those actions in the shop now, just installed a M70 safety to it and am barreling it to a 458 Win Mag for the customer

So very close to the current Zastava M70 bolt and cocking pieces

Also the FN cocking piece you show appears to be from a JC Higgins M50, as it has the angled sear face

Jim Wisner

thanks a lot Jim, I was pretty stumped by it and suspected a speed lock idea with nothing else I can see being gained by reducing travel.

As a separate note - the FN CP is off a Stirling, which was a name used by Fuller Firearms. It has a Sako #4 trigger, with a rather alarming slope on the sear where it contacts the cocking piece - is this normal?

21 March 2024, 00:50

brnomauser

quote:
Originally posted by Duane Wiebe (CG&R):
Have personally seen two of those "speedlock" cocking pieces fail (brake)

Either outrageously hardened or perhaps a casting..the nose is just hanging out there a bit far

thanks for the input