I have a question about your comment -
"This is walking the line as you need at least 0.002" longer chamber headspace than cartridge headspace for the cartridge neck to have room to expand and seal properly."
--------------------
I thought the 0.002 HS clearance was needed for reliable chambering and bolt closing & opening. Basically so there is room in the chamber from the shoulder to the boltface to accept the cartridge. I don't think that HS is involved with the neck measurement.
Neck clearance is determined by how that area of the chamber is cut - the chamber HS is rearward of the neck, from the chamber shoulder area to the boltface.
And typical 7.62 & 308Win chambers are cut with plenty of room for neck expansion.
Jay Kosta
Endwell NY USA
Hey Jay, you are absolutely correct that headspace (for this cartridge) is measured from the bolt face to the datum on the shoulder and the 0.002” clearance does aid in feeding and extracting.
I probably worded that wrong and should have left "neck" out of that sentence. Let me try and explain how I envision it for clarification and so someone can step in and correct me if I misunderstand the situation.
Let's consider the bolt action for a moment. In a bolt gun, handloaders will often have the case sized as close to the dimensions of the chamber as possible for accuracy and longevity of the case. Brass that has been fired in a particular rifle will often just be neck-sized as the case is fire-formed to the dimensions of that rifle’s chamber (minus maybe a thousandth where the brass retracts when cooling). I know I like to “feel” the tightness of my handloaded 308win rounds in my rem700 when I close the bolt. In this example the cartridge headspace is not 0.002” shorter than the chamber headspace and the case’s shoulder is tight against the chamber shoulder when the bolt is fully closed. When firing, the case becomes so hot its basically molten and most of the “flow” of the brass will show up as increased length in the neck area because the cartridge headspace can't grow any longer. This is not a dangerous condition in a bolt gun because the bolt is locked shut until you move it. No matter how fast you are on the cycling of the bolt the fired brass will be much cooler than the automatically ejected brass from a gas gun because it had significantly more time to cool in the chamber (and retract from its largest size). Since you are the operator of the bolt and cause extraction and feeding it does not matter if it takes 2 ozs or 20 lbs of torque to operate the bolt, it still can be operated without fail.
In the M14 there are a variety of parts which move at a particular time and speed with the pressure curve of the ammunition it was designed for in order for it to operate properly.
When any cartridge is fired the brass will “flow” towards the neck because that’s the path of least resistance (with the cartridge being supported in the rear by the boltface and on the sides by the chamber walls).
Imagine a bottleneck case a few thousandths shorter than the chamber being fired. The brass can expand ("flow") towards the neck before, at, or after the shoulder without much interference. Now imagine a case that has its shoulder tight against the chamber shoulder. It’s like the case is pinched at the shoulder, thus decreasing its ability to flow freely towards the neck. While this is desirable in a bolt gun it is not desirable in a M14.
In a boltgun the fired case has time to (relatively) cool down and retract from it's largest size and gas pressure is nonexistant by the time you can manually cycle the bolt. This is not the case in the M14, where it is rotated and begins to extract with gas pressures still present.
My understanding is that if there is not the clearance for the case to grow that 0.002” it may cause excessive wear on some of the rifle parts, possible extraction issues and may not make the gas seal properly. It may be that since the case wall is pinched at the shoulder and only the neck area can "flow", the brass may stretch from that point, get really thin there and even possibly break at that thinner point, as the hot case is violently twisted and yanked from the chamber. That's not good with the gas pressure present.
I hope that makes sense. Anyone should feel free to correct me or add to this. GI6
The Post and pictures are very well done, as is the narrative. Everybody that goes into this sort of modification should have a copy of your contribution.
There is little to be added to your Post other than to remind all that the primary thing to keep in mind is: Modify your rifle to fit the purpose of how you intend to use it..Do not come away from this Post assuming the lapping mod is for the purpose of setting Head Space to chamber a certain type of ammunition, that's a Sizing Die job..
The purpose of Lapping is to ensure both Bolts Lug contact receive recoil surfaces in an even manner to prevent undue stress on one Lug. Bolt Lug lapping is a "Safety" procedure first.. If you buy a Commercial M1A receiver this is something you should consider doing as the building process begins... Doing the Lapping on an assembled M1A requires knowing the existing numbers of the Head Space. There is no reason why you should not Bolt lap to get the benefits. primary and secondary, except the possibility of excessive Head Space resulting or cutting through the surface hardening.. Know your numbers before getting into Lapping.
This Gentleman has earned the second "Big Red" Award this week. Congratulations. Art
Thank you Sir! It is an honor to give a little something back as I have received much from those like yourself who are willing to volunteer time and effort to help bring others along. Besides, the documentation of my learning process helps solidify things in my own mind and gives opportunity to be corrected when wrong, or learn other, better ways of doing things.
Nice work!! Thanks for posting. Especially all the great pics. This thread has shown what you can do if you have the patience to work thru issues.
You mentioned that you can't finish it yet. What other projects do we have to look forward to?
Thank you. I have the Fulton receiver to build up also. I still have a Polytech that I need to do a GI bolt conversion on. Maybe eventually I’ll do a LRB M25 build.
On this LRB build I’ll need to get a knurling tool as the op-rod guide is loose. I’ll be trying my hands at bedding in the next months also. I’m sure other things will come up and I'm happy to document them for my own and for others' learning.
SS
