No.4 MkI can be fired form half cock

[Bruce_in_Oz]

If you have a "spare" sear, swap it in. If the "long arm" of the original sear is distorted, (bent), it may well be engaging the very end of the half-cock notch and this not locking up the bolt and trigger movement.

Swapping in an "unmolested" one is pretty easy, after removing fore-end first. You then have to attach the trigger-guard assembly in its correct location (sans wood, of course) and see what is happening.

Please keep us posted.

[Towarzysz_Beagle]

If you have a "spare" sear, swap it in. If the "long arm" of the original sear is distorted, (bent), it may well be engaging the very end of the half-cock notch and this not locking up the bolt and trigger movement.

Swapping in an "unmolested" one is pretty easy, after removing fore-end first. You then have to attach the trigger-guard assembly in its correct location (sans wood, of course) and see what is happening.

Please keep us posted.

That's a good idea, I happen to have a box of spare parts, I'll see if there's a sear in there.

[Towarzysz_Beagle]

If you have a "spare" sear, swap it in. If the "long arm" of the original sear is distorted, (bent), it may well be engaging the very end of the half-cock notch and this not locking up the bolt and trigger movement.

Swapping in an "unmolested" one is pretty easy, after removing fore-end first. You then have to attach the trigger-guard assembly in its correct location (sans wood, of course) and see what is happening.

Please keep us posted.

Late update, but you were spot on mate.

I took the forend off and attached the trigger guard and bushing. The sear is being caught by the half cock but not making it all the way into the notch.

The interesting part is that the original sear does not seem bent or damaged in any way. It engages properly with the bent, and disengages consistently when fired. It also engages correctly with the safety.

The rifle does have a non-matching bolt. Which bears well on both recoil lugs (first thing I checked on the rifle). I'm guessing this issue with the half cock is a matter of fit, and I am unsure how to correct it.

[Bruce_in_Oz]

Just wondering: Has it ever been a "Range" rifle?

Folks used to do some "unusual" things to their target Lee Enfields; some of them extremely dangerous. Triggers, sears and cocking-pieces got all manner of "tuning" done to them. Bolt bodies as well; really quite scarey...

Heard of "psycho-acoustics"? I am fairly convinced there is a school of "psycho-ballistics" as well.

[Towarzysz_Beagle]

Just wondering: Has it ever been a "Range" rifle?

Folks used to do some "unusual" things to their target Lee Enfields; some of them extremely dangerous. Triggers, sears and cocking-pieces got all manner of "tuning" done to them. Bolt bodies as well; really quite scarey...

Heard of "psycho-acoustics"? I am fairly convinced there is a school of "psycho-ballistics" as well.

Oh I'm no stranger to this. I've spent a lot of time shooting and collecting Enfields. I have come across some pretty crazy bubba examples in this time.

Like a trigger where the humps had been so haphazardly ground down to create a single stage, that the end result was the sear protrusion into the bolt channel being inconsistent and not always catching the bent. Couple this with when it did catch its position was so low on the bent that if you gave the ground a tap with the but it would release as if the trigger had been pulled. That was extremely dangerous obviously but also easily fixed with a new trigger.

I've also come across ground down bents as well which again interfered with the consistency and safety of its engagement with the sear.

So yeah I have a fair idea of what to look for with some of the dodgier things people do and this rifle doesn't exhibit any symptoms of tampering/grinding etc on the bolt, sear or cocking piece.

It's really got me scratching my head.

[englishman_ca]

If somebody stoned the bent on cocking piece, the very edge, from which the sear trips, will have changed position.

The metal has a sloping surface behind the bent where it is cut for the half cock notch. If you remove metal from that face, you will start moving up that ramp with the edge. The end result is that the height changes. If stoned enough, the tip of the sear when it trips is no longer at a height to clear the half bent and will catch on the half cock, either hanging up, or causing the striker bump and to slow down, resulting in a light primer strike.
Usually discovered when squeezing the trigger very slowly to release. Cant tell by a quick function check. A tug on the trigger, everything still works as it should. A squeeze, it can jump into half cock, or in this case, hang on the edge..

Often seen on Farmer Brown trigger jobs, his solution would be to file a bevel on the underside of the cocking piece at the half bent to allow the sear to slide past.

It gets worse as Bubba tries to fine tune it with file and stone.

No fix for this other than replacement. On inspection, if I see file and stone marks on the cocking piece, it gets switched out. No point in trying to fix.

Perhaps this is not the exact situation here, but it is all related. The trigger group is a tuned system. Adjust one thing and you screw up another.

My thoughts are to simply change out the cocking piece with a known good one and maybe the sear, that should certainly fix the problem.