Gunsmithing Q and A

with Master Gunsmith and
AGI Instructor Ken Brooks

Gun Club of America members get special help from the AGI instructors as part of their membership, including the “Ask The Pros” forum on the members-only website. Here are just a couple of the thousands of great tips that have been answered by Ken Brooks.

Glass Bedding

Question:

Gene Shuey’s video on glass bedding was very good, but he did not explain the benefits of pressure bedding even though he mentioned he preferred it over other methods. Can anyone tell me what those benefits might be?

Also: I took in an old bolt action 20 ga. shotgun repeater that was in “well used” condition. It is a Sears Roebuck Ranger 141-7. I found that H&R had a similar model # 121, but there are not any parts available at Jack First or Numrich. The part I need is the carrier spoon. It looks like it is a part that acts as the secondary cartridge stop. If I knew what they looked like I could probably make one. Anyone got any info on this gun and or parts Answer:

I don’t know why Gene prefers that method. I prefer whatever works. Some barrels like pressure and some do not. I like fiberglass stocks for many reasons and you cannot pressure bed correctly with fiberglass or plastics, so if you have a gun with a small barrel dia and it likes pressure you would have to have a wood stock.

Answer:

The easiest way to do this is, as Ken says, see how it shoots if you free float it. Then stick a couple of business cards between the forend tip and the barrel, trying the groups as you add each card. If this makes for better groups, turn the gun upside down and clamp it into a vise. Pull down on the end of the barrel with a trigger pull gauge and note the weight registered at the point that the card(s) pull out easily. Then, as I said in my 10/22 Tips and Tricks article in GunTech vol 5, “you can also color up some of the ACRAGLAS gel to match the stock and lay about a 2” band in the barrel channel from the fore end tip back. Use the gel so it doesn’t run out because we’re going to put the stock upside down (horizontal) in a vise and hang a 1 fi-2lb weight (in your case, whatever weight the trigger gauge showed) from the muzzle. I use a big crescent wrench with a shoestring through the handle hole (very hi-tech and scientific) and hang it over the muzzle. When the glass sets up, you will have an upward pressure at the fore end tip equal to the weight of the wrench (or other weight). Try the gun before you do this and only do it if it needs help.”

Rimfire Offset Firing Pin

Question

I am working on a Cobra Derringer .22lr. The firing pins are long (.124 on top and .113 on the bottom, max and absolute protrusion) and have dinged both chambers. The firing pins are also offset (you can’t just chuck it up in a drill and put the hemisphere on it). I also do not have a four jaw lathe chuck to put it in either. Any ideas or simple fixtures that might work?

Answer

Vice and files and sand paper then buffer. Remember on a rimfire it doesn’t have to be hemispherical. Ken

Answer

Don’t fire rimfires if absolute protrusion exceeds .035”. Get a Menck rimfire chamber swage from Brownells to remove dings, or make your own. Jack

AR-15 Barrel Dimpling

Question

I need to install a low profile gas block. It seems most come with set screws. Is there a standard depth the barrel should be drilled to? I assume the dimples should be ever so slightly larger than the set screw. Any tricks for lining up the gas port?

Answer

You will want the dimples in the barrel to be smaller than the screws so the tip of the screw will engage the dimple and keep the whole affair tight. If the dimples are oversized then the screws may not keep it tight. As far as indexing the hole, mark the top of the barrel where you will be able to see it with the gas block in position. Then line up the gas block as needed with your index mark. Ken

Answer

Make pencil line on barrel in-line with gas port. Index gas block with line. Point up screw that fits set screw holes. Screw in to mark center point of dimples. Use a sharp drill slightly larger in diameter than the screws. Only drill deep enough to make the dimple slightly smaller in diameter than the screws. The bevel on the screws will then center in the dimple. Jack


8 Responses to Gunsmithing Q and A

  1. Good answers to questions. Brownells has discontinued their Menck tools. Liked the design as they still had a picture. Others seemed to be available.
    Could you define, “Don’t fire rimfires if absolute protrusion exceeds .035”.”
    Thanks.

    • Hey Danny,

      I think Ken means “don’t ‘dry fire’ rimfires if absolute protrusion exceeds .035″, so I will elaborate on that notion. With that, you may already know what I am about to say here, but if by chance you do not then here it is:

      A rimfire firing pin should not be able to protrude out from the face of the bolt no more than .035”. That is what Ken means. Bob Dunlap recommends the firing pin not to exceed .040″ protrusion (so it doesn’t dent the edge chamber mouth) and must protrude a minimum of .025″ (for reliable ignition purposes) but Ken obviously has his own maximum protrusion recommendation. I think Ken is being on the totally safe side of things with a maximum protrusion recommendation of .035″ .

      Basically, by adhering to the maximum firing pin protrusion spec of Ken’s .035″ maximum then this will avoid the chance of the firing pin from being able to hit the chamber when dry firing the gun on an empty chamber.

      The way to check for this is to remove the bolt, push the firing pin as far out as it will go (this is called “absolute protrusion”) and measure (in thousandths of an inch) how far the pin protrudes outwards from beyond flush with the face of the bolt. This is your absolute firing pin protrusion measurement. Just make sure when checking for absolute protrusion on spring loaded firing pins to make sure the pin is pushed out as far as it will go and then take the measurement.

      As a side note, if the firing pin return spring is robust it may be difficult to coil bind the spring which is needed to get a proper absolute protrusion measurement. I find non-spring loaded pins are easiest to measure because of that.

      I highly recommend doing this for each of your rimfire guns and make a note. Then file the note(s) for each gun so you have future reference. You can place the note in the butt of the gun if you prefer that for quick reference in the field or whatever. I make my notes on computer but am thinking I should make a note sheet for each gun and seal it in a ziplock bag then put it in every one of my gun’s butt-stocks.

      What it really comes down to for rimfire guns is if the firing pin tip can protrude past the end of the BOLT BODY (or bolt nose, whatever you may call it). If it protrudes past the end of the bolt body then the firing pin will contact the edge of the chamber mouth and most likely put a ding/dent at that area of the chamber mouth. This happens because of the rimfire design. Rimfire firing pins hit on the edge of rims thus hitting in line with the edge of the chamber mouth. If it can’t/doesn’t protrude past the end of the bolt body then you’re in the clear for dry firing.

      Centerfires are of no worry for this because the firing pin is centered in the bolt face and thus centered with the chamber, hence there is no chance of the pin being able to hit anywhere near the edge of the chamber – the pin will protrude in centerline with the center of the chamber. There’s no metal for a centerfire firing pin to hit/contact when they are dry fired.

      Hope this helps. Cheers.

      • I would like to add about rimfire guns that don’t have a last round bolt hold open feature and have a firing pin that can protrude past the end of the bolt body/bolt nose. If the shooter is not counting (or lost count) and dry fires on an empty chamber they are at high risk of denting/damaging the edge of the chamber mouth. For these guns it is essential to check for firing pin protrusion before using the gun.

        Some firing pin tips surfaces are uneven. I had one gun in for repair that had the top of the rectangle firing pin tip longer on the top than the bottom . The top portion of the pin tip had dented the chamber. It was a lever action gun (in which of course does not have a last round bolt hold open feature) in which if shooters lost count or were not counting then the gun was dry fired.

        So, when measuring for protrusion measure up and down and side to side and take the longest measurement, or level the end of the pin’s tip so the measurement is consistent across the entire tip’s end surface.

      • Yes, understand, your answer is not what I was thinking about. I was thinking about the shell in chamber, not the firing pin. Seem to remember Bob saying .040.
        Thanks.

          • I could not figure out what the wording meant. I just had a Winchester 255 that came into my shop with firing issues. An issue in the gun I received was the shell going too far into the chamber. So, somehow, I was relating the .035 to chamber and shell rather than firing pin. My fault.