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Discussion Starter · #1 ·
As I become more attuned with my mini I can hear and feel the action ringing like a gong with every shot. This might just mean that the gas block buffer needs replacing but I can't help thinking there is more to it than meets the ear.

With the resurrection of Ed Harris' article the idea comes back to the slide whacking the gas block. It would seem that most feedback advises against Mr. Harris method for stopping the slide at the bolt lug. The more I think about it the better it sounds.

Inspecting the 196 showed that the rib on the slide is indeed contacting the gas block and a new buffer is needed. Before doing this I hand cycled the rifle without the buffer and took a few measurements. The rib on the face of the slide measures .055". I stuck a seperator into the gap between the lug inside the cam pocket and to the best of my abilities it measures +/- .050".

In the next few days I am going to machine the face of the slide. If it doesn't work I could put the buffer back in.
Guido
 

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I've only been around here since well into the 580 series, so I haven't seen anyone do the Harris mod. I've read a lot about it and heard it mentioned, though.

Curious to see if/how it improves things on the pre-580 Mini.
 

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Guido, this is going to sound arrogant but bear with me...I believe I know what your thinking and I would advise against it "somewhat". Here goes. This "Harris Mod" IMO was a big theory that wasn't followed up entirely in practice. You can surely square up the end of the op rod and it won't tear up your buffers. My op rod was out of square so bad I had to grind down more than just the "ribs". Here's where it gets interesting. After grinding, the op rod no longer hit the gas block but bottomed out on the end of the piston; it wasn't making it to the buffer. Now what! Shorten the piston or drill a deeper hole in the op rod? Here's where I decided to test the rest of the Harris theory. I measured the gap in the cam slot as you did and came up with .050". I didn't care what I messed up at this point so I chose to grind down the piston to close the gap at the cam. I got it down to .020" and that's as far as it would go. Seems the slot needs to be deeper for the cam at that point. The cam and slot are not machined to fit that way even though it will try to lock up somewhat. Good news is the buffer was doing is job again. IMO it would take a redesign of the gun to make it work like Harris wanted; which I think is what he wanted! Be careful when you square it up and you'll be fine. Just don't try to close the gap, it's not gonna work the way you might think. Mine is a 581 and who knows how close it is to the other models as far as tolerances of parts. Btw the gun IS a lot smoother now but I'm gonna need a smaller gas bushing...again. If any of this wasn't in your plan then please ignore it.:)
 

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wish this site supported a "rep" function like some others did -

Whillman - I'd have thumbed up your post for some positive rep.

well put, sensible post with a nice "if you already thought of this then no worries" attitude. from a dude with less than 100 posts no less.

have a beer :lol:

:beer:
 

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Discussion Starter · #6 ·
whillman,
Thanks for the feeback. It hadn't dawned on me the the bolt lug might not travel fully to the rear of the cam slot.

Let me see if I understand what you wrote.
You ended up with a .020" gap between the gas block and slide because the bolt lug was binding before it travelled fully to the rear of the cam slot.
I will check this by removing the gas block.

I only intend on removing the rib and just enough to true the slide face. Possibly brazing a piece into the cam slot would hold the slide off the gas block.

Guido
 

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Guido,

My unmodified black 580 Mini has the oprod stop on the receiver, not the gasblock. How do I know? The oprod lug that slides in the receiver and the forward ledge of the receiver track peened each other, while the gasblock has no marks on it where one would expect it. The front of the oprod face is similarly almost untouched in appearance.

The oprod is stabilized by this hard contact with the receiver track when fully forward - even after I dressed off the burrs that were kicked up.

Note that the oprod does not stop on the bolt lug - the cam does not push the bolt further forward after rotating it into the fully locked position.

The oprod on my newer stainless Mini has loads of side-to-side slop when fully forward: It takes little force to make it rock left to right when fully forward and the resting place seems potentially ambiguous. This, while the black Mini oprod snaps back to the same position if deflected by finger pressure.

Based on wear marks, the oprod on the stainless Mini was making uneven contact with the gasblock: I then milled the oprod face off to even out its contact with the gas block by removing 0.02" measured from the highest point.

I have re-cut the step in the oprod face, so other than that it is squared up, it looks factory, not hacked. The intent of the step is to vent gas mostly down when the gas pipe first emerges from the hole in the oprod face.

It is possible that the stainless oprod is now also stopping on the receiver - I plan to cut a little flat on the rounded tip of the oprod lug so that it would make contact first with the upper and lower edges of the semi-circular cutout in the receiver.

The idea is to create a V-block effect that would hopefully cause the oprod to stop in the same position left to right each time. Why is this important? It might have the potential to change the vibration profile shot to shot.

If I did not have the black Mini as a reference I might conclude that the slop in the stainless oprod track is normal….
 

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whillman,
Thanks for the feeback. It hadn't dawned on me the the bolt lug might not travel fully to the rear of the cam slot.

Let me see if I understand what you wrote.
You ended up with a .020" gap between the gas block and slide because the bolt lug was binding before it travelled fully to the rear of the cam slot.
I will check this by removing the gas block.

I only intend on removing the rib and just enough to true the slide face. Possibly brazing a piece into the cam slot would hold the slide off the gas block.

Guido
The gap I was measuring was between the rear of the cam and the end of the op rod slot. If the slot was machined to match the cam lug profile I'm sure it would fully lock down. Would it help? I don't know but I have my doubts. My intention was to have equal pressure on the cam lug and the gas block but it would take a lot of trial and error....for me anyway. Plus I'm not sure if the cam lug could take the stress. The gas block is pretty robust and seems it was meant to take the brunt of everything. I'm sure you know what your doing; you can easily square up the end of the rod. But like I said mine was in bad shape and I had to take off more than intended. Hope I'm not confusing you. There are lot of guys here more knowledgeable about this than me, I was just trying to keep you from a possible rabbit trail. Let us know how it works out.;)
 

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wish this site supported a "rep" function like some others did -

Whillman - I'd have thumbed up your post for some positive rep.

well put, sensible post with a nice "if you already thought of this then no worries" attitude. from a dude with less than 100 posts no less.

have a beer :lol:

:beer:
Thank you for the kind words Snuff. Yeah, I wish we had that feature too because I've read a LOT of GOOD info on here from time to time. Not saying my info is good but just saying.;)
 

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I'll suggest first before you do the machining to tap the gas pipe for a short set screw to make your Mini a straight pull bolt action - this will tighten groups some if that's your intent and it's easily re-verted back. the pipe is hard stuff so caution is called for.
this may provide some more insight into your mod. myself I think both head space and chamber dimensions are set up a bit sloppy for reliability sake which is going to limit the effectiveness of attempts to tighten things up.
I'd suggest a trigger job, stock/receiver beddding/shimming, gas block bedding, a good check of the muzzle crown and installation of an effective muzzle brake - this is what I did to my 189 series M30 and halved groups from factory - which was effective enuff for deer hunting but my good handloads with 150gr Speer sp have made a more effective rifle for larger whitetail, muleys and if I ever get a chance Elk and/or Moose.
it's mostly shot placement anywhos you know.
welcome to the Mini pages from a Western North Carolina member.
 

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I milled my op-rod ridge down to make it contact the gas block evenly, and to hopefully tighten the bolt lockup. I got it squared up nicely with just .012 off. I didn't go any further because there is so much slop at the bolt lug, even taking it off completely would not make it tight. I hadn't though about the piston hitting bottom.. I think you could turn or grind some material off the end of the piston if you had to.
 

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I guess more questions here than answers, but here goes...

It looks like it may also be feasible to drill and tap the rear of the op-rod for a set screw. The hole would be at the rear of the op-rod, where your thumb might rest while grasping the handle of the op-rod. Then you could adjust how much pressure is applied to the bolt's stud, and how much pressure is shared with the gas block. Also easily reversible; just take the set screw out.

Thinking more about this last night, I began to question (as a thought experiment only) if this would bring accuracy gains... Wouldn't the pressure from the op-rod to the bolt unevenly cock the bolt sideways? I.e., the right-hand locking lug could be lifted slightly, while the left-hand lug would be carrying all the load prior to ignition.

This might increase the variables of shell alignment within the chamber, including:
- Brass shoulder to base length variances, leading to inconsistent locking lug balance;
- Variances in bolt/op-rod bounce, also inconsistent to lug balance.

The flip side is that spring pressure may make a minimum headspace condition every time, thereby increasing accuracy.

Yet another thought I had from this, if the gas block is to be used for op-rod contact, requires the rifle to a tapered barrel Mini (mine isn't...) Would truing the forward edge of the gas block halves, like truing a receiver face, bring benefits? Make a fixture to bolt the gas block halves onto, chuck the fixture in a lathe, and face the forward edge. This would bring even pressure to the gas block, and reduce variables.

Thoughts?
 

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Oops! I forgot to mention that my comment were not meant to talk you down from this project; I'd love to see what the reality of this mod is.

Now go make some metal chips! ;)
 

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scottm343, I agree totally with your assessment. If you had seem the end of my op rod you would wonder how it left the factory. I had to take .040" off to get it square. That's a nice job you did on yours. I wish I still had access to a mill and lathe. Nice pic.
 

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Wouldn't the pressure from the op-rod to the bolt unevenly cock the bolt sideways? I.e., the right-hand locking lug could be lifted slightly, while the left-hand lug would be carrying all the load prior to ignition.

This might increase the variables of shell alignment within the chamber, including:
- Brass shoulder to base length variances, leading to inconsistent locking lug balance;
- Variances in bolt/op-rod bounce, also inconsistent to lug balance.
This is exactly the reason why you don't want to apply any external force to the bolt while it is in battery, other than keeping it rotated into full lock-up. The oprod cam slot does not push the bolt forward once the bolt has been rotated into lock-up.

The bolt needs to be able to float so that it is self-aligning: Alignment comes from the round in the chamber and even contact on the locking lugs. At the rear of the bolt, there is very little left to right alignment force generated, mostly vertical with the back of the bolt forced into contact with the curved receiver ceiling by the magazine spring and hammer nose (on firing).

The flip side is that spring pressure may make a minimum headspace condition every time, thereby increasing accuracy.
Pushing the bolt forward so the round is in hard contact with the tapering portion of the chamber at the shoulder may help center the round. However, at the instant of firing, the bolt's locking lugs will move back into contact with the receiver lugs - if the gap that closes is significant, so is the velocity of the bolt, slamming the lugs together. It would be better for the locking lugs to already be in contact at the instant of firing. In other words, the cure may exacerbate the slop you were trying to fix.

The bolts on both my Minis seem quite sloppy with lots of "headspace". That is until you close the bolt on a loaded round and feel how snug the bolt is - try this with the rifle mostly disassembled (oprod and trigger assembly removed).

With a round in the chamber, the only part of the bolt that moves somewhat easily is at its rear end - the bolt alignment driven by the loaded round causes the rear of the bolt to "lift" into contact the receiver ceiling, but the bolt can be pushed down at the rear due to the leverage afforded by the bolt length. However, in practice the magazine spring is pushing the rear of the bolt up - the force is transmitted by the rounds in the mag and by the follower on the last round. Also, there is no force pushing the rear of the bolt down except for half the weight of the bolt - easily overcome by the magazine spring.

As an aside, I am reading Hatcher's book on the M1 Garand development. The chapter that deals with headspace said that there was no observable difference in accuracy with headspace set to minimum or at the field maximum - a range of 0.010" (.30-06 caliber).

What this suggest is that you could explore hand loaded ammo that has been neck sized only or where the body is resized just enough to allow the bolt to close and see if this improves accuracy - if not, then modifying the bolt with set screws is not going to help.

One thing all of you tinkerers need to remember is that if large forces are applied to small areas, metal will be displaced due to the contact pressure exceeding the strength of the material. In other words, be careful where you place set screws and consider the loads you expect them to carry - especially repetitive suddenly applied loads.
 

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Discussion Starter · #16 ·
Great feedback.
I started milling the slide and ended up removing .075" off the face. Shortened the gas piston .050" and turned a 60deg taper on the end. This netted a .003" gap at the gas block with the slide catching in the end of the track in the reciever.

I have hopes that this will bring some improvement but only time will tell. I am sure that the lug in the receiver track will peen a bit. Worse comes to worse I can put the buffer back in.

Guido
 

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This is exactly the reason why you don't want to apply any external force to the bolt while it is in battery, other than keeping it rotated into full lock-up. The oprod cam slot does not push the bolt forward once the bolt has been rotated into lock-up.
That's pretty much where I had come to while thinking all of this through.

Any thoughts on facing the front of the gas block on tapered-barrel Mini's? (Provided, of course, that the gas port relationship didn't get screwed-up...)

Again, wouldn't apply to my Mini; just another thought experiment. (And if too off-topic, just smack me!)
 

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Great feedback.
I started milling the slide and ended up removing .075" off the face. Shortened the gas piston .050" and turned a 60deg taper on the end. This netted a .003" gap at the gas block with the slide catching in the end of the track in the reciever.

I have hopes that this will bring some improvement but only time will tell. I am sure that the lug in the receiver track will peen a bit. Worse comes to worse I can put the buffer back in.

Guido
Any pics? And I can't wait to see results of your work, positive -or- negative!
 
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