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There are several types of Guides now available for the M1A. The difference is in length and quality of fit, to both the barrel and the Opt Rod. When you buy a Guide, take your Opt Rod and Barrel, if possible, with you, so you can check for a smooth slip fit to both.
The purpose of the Guide is to stabilize and guide the Opt Rod during the rear and forward travel when the rifle is fired. Also, to align the forward end of the Rod with the end of the Piston Stem, if you do not have one of my Alignment Fixtures, you should, which protrudes out the rear of the Gas Cylinder and is in contact under some pressure from the recoil spring..There must be some pressure between the end of the Opt Rod and the end of the Piston. Both of these requirements are equally important for function and absolute for Accuracy.
The following is a personal opinion on how to best mount an Opt Rod Guide, this method applies to the USGI Guide and all others. It is not a mandate.
The USGI Barrel has a raised Collar to accommodate the USGI Opt Rod Guide, also the USGI Barrel has a slot on the underside to accommodate the Guide Pin which helps to hold the Guide to the Barrel and to prevent rotation or front or rear movement. Some Commercial Barrels have this Collar some do not, some have the Pin slot some do not. The mounting surface on the Commercial Barrel is machined longer to offer the longer purchase area of a longer Commercial Guide, and usually a little oversize in relation to the passage in the Commercial Guide for a tight fit.
Which ever type of Barrel or Guide is to be used, except for a long Commercial Guide on a USGI Barrel, which I have not as yet seen, the fit between the two parts can be achieved in a number of different ways, The most common way is a DRIVE on fit, this requires the use of a hammer of some sort and a tool to place against the Guide to prevent damage to the Guide and to force the Guide on and over the Barrel or Barrel shoulder. This has been the standard method of installing the Guides for many years. Barrels that have a mounting surface or Collar that is too small, wherein the Guide to be used is too loose, can be knurled or stippled to raise up the mounting Barrel surface to take up the slack is a common practice, and a good one.
I prefer to do it differently, I do not want any possibilities of any constrictions on the Barrel.
Also, I do NOT want to to install anything on the Barrel that requires a hammer to get it on. That sentence is the essence of this Thread. This is not the method I was taught originally, but in my experience, in search of Accuracy, another mounting choice, exactly the opposite resulted.
This is what I call the Slip Fit Method, used between the Guide and the Barrel. If necessary, I remove metal from either, or both, the Barrel or the inside of the Barrel mounting hole in the Guide to get a Slip on fit of the Guide over the Barrel. This means, when the Guide is selected, I fine one that is as close to the Slip on fit as possible, or a second choice of, selecting a Guide just a little too large, but not sloppy. The second choice requires a thin strip of brass shimming material placed at 12;00 o'clock on the Barrel when I epoxy the Guide on. It is unusual for me to use a Guide Pin at all.
Is this time consuming, yes it is, is it necessary, I think so, does it improve accuracy? What does not improve accuracy is not taking enough time when doing a installation, that on the surface, it appears simple.. Art
The purpose of the Guide is to stabilize and guide the Opt Rod during the rear and forward travel when the rifle is fired. Also, to align the forward end of the Rod with the end of the Piston Stem, if you do not have one of my Alignment Fixtures, you should, which protrudes out the rear of the Gas Cylinder and is in contact under some pressure from the recoil spring..There must be some pressure between the end of the Opt Rod and the end of the Piston. Both of these requirements are equally important for function and absolute for Accuracy.
The following is a personal opinion on how to best mount an Opt Rod Guide, this method applies to the USGI Guide and all others. It is not a mandate.
The USGI Barrel has a raised Collar to accommodate the USGI Opt Rod Guide, also the USGI Barrel has a slot on the underside to accommodate the Guide Pin which helps to hold the Guide to the Barrel and to prevent rotation or front or rear movement. Some Commercial Barrels have this Collar some do not, some have the Pin slot some do not. The mounting surface on the Commercial Barrel is machined longer to offer the longer purchase area of a longer Commercial Guide, and usually a little oversize in relation to the passage in the Commercial Guide for a tight fit.
Which ever type of Barrel or Guide is to be used, except for a long Commercial Guide on a USGI Barrel, which I have not as yet seen, the fit between the two parts can be achieved in a number of different ways, The most common way is a DRIVE on fit, this requires the use of a hammer of some sort and a tool to place against the Guide to prevent damage to the Guide and to force the Guide on and over the Barrel or Barrel shoulder. This has been the standard method of installing the Guides for many years. Barrels that have a mounting surface or Collar that is too small, wherein the Guide to be used is too loose, can be knurled or stippled to raise up the mounting Barrel surface to take up the slack is a common practice, and a good one.
I prefer to do it differently, I do not want any possibilities of any constrictions on the Barrel.
Also, I do NOT want to to install anything on the Barrel that requires a hammer to get it on. That sentence is the essence of this Thread. This is not the method I was taught originally, but in my experience, in search of Accuracy, another mounting choice, exactly the opposite resulted.
This is what I call the Slip Fit Method, used between the Guide and the Barrel. If necessary, I remove metal from either, or both, the Barrel or the inside of the Barrel mounting hole in the Guide to get a Slip on fit of the Guide over the Barrel. This means, when the Guide is selected, I fine one that is as close to the Slip on fit as possible, or a second choice of, selecting a Guide just a little too large, but not sloppy. The second choice requires a thin strip of brass shimming material placed at 12;00 o'clock on the Barrel when I epoxy the Guide on. It is unusual for me to use a Guide Pin at all.
Is this time consuming, yes it is, is it necessary, I think so, does it improve accuracy? What does not improve accuracy is not taking enough time when doing a installation, that on the surface, it appears simple.. Art