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Thread: Fire lapping a barrel using valve grinding compound.

  1. #1
    Boolit Master
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    Fire lapping a barrel using valve grinding compound.

    I've been thinking about fire lapping the barrel of my 1892 Winchester .32-20 since I get some rounds that keyhole, especially with the factory ammo, and there is some pitting. I know I don't have a huge amount to gain, but maybe if I can get a little better accuracy out of it, I will be happy. I slugged the barrel, and I get .312" so I still have rifling left to work with. I see quite a few people recommending valve grinding compound, just wondering if someone who has used that method, how many bullets have you fired out the gun doing it that way?
    Here's what one group looks like with my 115 grain handloads and .312" bullets.

    This is at 25 yards, I would say the group is about 2 inches.



    Now here it is with factory Remington 100 grain bullets. I don't know what size the bullets are with this ammo if they are a smaller diameter but they were going all over the place and keyholing all the way there. Same distance. Not one of them hit the target with me shooting from a rest.

    No people in the world ever did achieve their freedom by goody-goody talk and moral suasion: it being immutable law that all revolutions that will succeed must being in blood, whatever may answer afterward.

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  2. #2
    Boolit Master
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    I totally ruined a barrel with valve grinding compound. I would try paper patching lead bullets. the paper will shine a barrel and I would look at the crown on the muzzle. It may need recrowning. You probably have .308 jacketed bullets. You may be getting some upset with the lead and it's filling the bullet out to fit the bore. No way would I use valve grinding compound. There are people on here that know more about paper patching than I do, but basicly you cut a strip of paper with an angle cut on the end, make 2 wraps and cut a matching bevel, leaving a tail you can twist to hold the paper on. wet the paper and wrap the bullet tightly, and twist the tail. You can lay the bullet on its side and roll it and tighten the wrap. let the paper dry, you can trim the tail. You don't need lubricant. Depending on the paper thickness, you need a bullet about .300, or .302, to wrap

  3. #3
    Boolit Master
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    Fire lapping sometimes work but as suggested earlier paper patched bullets might be better choice for a first try. I have manually hand lapped a lot of barrels over the years but I ruined a few barrels in the process of learning the process.

  4. #4
    Boolit Master
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    The least expensive thing to do is pull and measure a factory bullet. If it is undersized for your bore then no amount of polishing is going to improve the situation in a meaningful way.

    If I was in your shoes I’d try sizing your cast 115g bullets at .313 and .314 and see what happens.

    IMO it is never wise to do something irreversible as a first step.

  5. #5
    Boolit Master Hannibal's Avatar
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    ^^^^ This. ^^^^

    Very sound advice.

    If your bore measures .312", best results are typically found .001" to .002" oversized. So .313" to .314" as mentioned above should work well.

    And again, as mentioned above, the keyholeing is likely due to the jacketed bullet measuring several thousands under the bore diameter. You won't have any stability if your barrel can't impart any spin on the bullet, and the only way to know for certain is to pull a couple down and measure them. BTW - these tasks are better performed with a micrometer and not a dial caliper. But if a caliper is all you have, that measurement is better than no measurement.

    I'd steer clear of valve grinding compound. You'll likely wipe any leade that barrel has left out in a very few firings. Valves and valve seats are a lot harder than a rifle barrel. Wrong abrasive for the application.

    The crown is another area that could be causing trouble. When you slug your bore, carefully check the muzzle end to see if it is a looser fit on your bore slug than the rest of the bore. This requires.a steady hand as you have to go by the 'feel' of the slug in the bore as you slowly and carefully push it to and fro in the barrel.

    I suggest you start with cast in the .313"-.314" range and measuring your jacketed bullets. Changing only 1 thing at a time is the best approach.

    Good luck!
    Last edited by Hannibal; 08-07-2018 at 10:18 AM.
    Missing the target is not the worst thing you can do.
    Not taking the shot is.

  6. #6
    Boolit Master


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    I have fire lapped dozens of barrels and ALWAYS got an improvement in accuracy! I use JB compound or NECO bore polish grit. 600 grit valve grinding compound might work if you can find it. David Tubb sells a kit with 5 or 6 different grit plated bullets which I have also used with great success. They are available in different calibers and sold by Midway. What ever one uses the key is not to use a course grit.

    Larry

  7. #7
    Boolit Master Hannibal's Avatar
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    I wouldn't attempt to lap a barrel and increase the bore diameter if I were having keyholing issues with factory ammo until I knew WHY.

    I wonder how many times the accuracy improvements seen after fire lapping are due to excessive carbon or copper fouling that was never correctly identified and removed with less aggressive approaches?

    YMMV.
    Missing the target is not the worst thing you can do.
    Not taking the shot is.

  8. #8
    Boolit Master

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    Valve grinding compound is a bit aggressive for this application. This is what most use https://www.midwayusa.com/product/21...-compound-1-oz. I'm not a big fan of lapping a barrel but some times it helps. When all ells fails it's time for a liner.

  9. #9
    Boolit Master
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    There are different grades of valve grinding compound. When doing heads, I have some that works well, some that is more like rocks in greas, and then I have some that is too fine for seating valves. I don't know what the grit is without going to the shop, but I know they are all the same brand. I would be hesitant about using any but the very finest I could get in a barrel. Even then, I don't know if I would try it.

  10. #10
    Boolit Master Hannibal's Avatar
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    There is also such a thing as polishing a bore too smooth and creating copper fouling problems. Significant fouling problems.

    Not sure if it has the same affect on cast bullets, haven't had a chance to experiment yet and haven't heard anyone else comment on it.
    Last edited by Hannibal; 08-07-2018 at 01:10 PM.
    Missing the target is not the worst thing you can do.
    Not taking the shot is.

  11. #11
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    Lapping will not cure key holing projectiles.
    I would say your bullet is under size. Remember, you are fitting to the throat, not the bore. I like my bullets to slip into a fired case with slight resistance. This should put you awful close to the right size.
    I agree with others, valve lapping compound would be too course for this purpose. I do use it to lap muzzleloader barrels, but the fit is not nearly as tight as a bullet is when lapping.
    The solid soft lead bullet is undoubtably the best and most satisfactory expanding bullet that has ever been designed. It invariably mushrooms perfectly, and never breaks up. With the metal base that is essential for velocities of 2000 f.s. and upwards to protect the naked base, these metal-based soft lead bullets are splendid.
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    Forget everything you know about loading jacketed bullets. This is a whole new ball game!


  12. #12
    Boolit Master Outpost75's Avatar
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    I would clean the bore well with Kroil and JB paste, then use a corect bullet of adequate diameter that fits. Accurate 31-105T works well with 7.5 grains of #2400 in my .32-20s.
    Last edited by Outpost75; 08-07-2018 at 12:32 PM.
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    HE wants to know what YOU know.
    Keep it to yourself.

  13. #13
    Boolit Master

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    I roll the as-cast (ideally slightly oversized) unlubricated boolits between two steel plates or strips with a dab of Clover 320 on the bottom plate, to press the compound in. The paste does not necessarily have to fill the lube grooves, but if it does, it doesn’t seem to hurt. I generally make up 50 of these lapping boolits and put them into one of those Styrofoam bases that come in 50-rd pistol cartridge boxes.

    I take the box, a couple brass cases, primers, Unique powder in a measure, a re-/de-capper, tweezers, a dowel that fits the chamber, a cleaning rod, brass brush, lots of cleaning patches and solvent, some rags, Q-tips, a bore light and a box of the best-performing loads for that rifle to the range and set up targets at 50 yards.

    I use one case, if at all possible; decapping, recapping and filling with a light powder charge. This will be a “sacrificial” case, as it will wind up heavily contaminated with the lapping compound. It will ruin your dies if you accidentally get it back into your regular cases.

    I put a boolit into the chamber with the tweezers and press it home with the dowel. Place the charged case behind it, close the action, and fire. Extract slowly so lapping compound doesn’t get into the mechanism, wipe down case, reprime and charge. Shoot four more times, removing any compound I can see in the action.

    After five shots, I clean the barrel with brushes and solvent. Remember the worst parts of the barrel will pick up the lead first, which will protect those areas from lapping by subsequent shots. Keep the barrel clean, so the pits and roughness polish at the same rate as the rest of the bore.

    After cleaning, I fire five shots of the “best” load and check the group size. Then I do five more lapping shots, clean, and test again. The powder charge for the lapping rounds doesn't need to be much more than enough to get the boolit out of the barrel, but I generally shoot these at a target too, just to see.

    If everything goes in the “classic” manner, I generally see the groups of the “good” load get smaller, and as the process continues, the cleaning gets easier as well. I mostly do single shots and bolt guns where I can clean from the breech. A 92 might need a muzzle protector and a means to hold the rifle upside down for cleaning, or tools to remove and replace the bolt. Make sure all of the compound is out of the bore, chamber and action works before firing the test rounds.

    I keep watching the test groups. If the improvement levels off, I usually stop there and fire the rest of the “good” ammo. I note that the barrel heats with the fire-lap rounds and will also heat more than normal with the first ten or so “good” rounds. I would guess that this is friction from the remaining compound being carried out of the bore by the regular boolits.

    It’s kind of a judgement call. If I’m happy with the results after 20, 25, 30 or 50 shots, I stop there. If subsequent range trips indicate that more lapping might help, I’ll do another session. Usually, 100-125 lapping rounds, max, over several sessions, wind up “as good as it gets” for me.

    I hope I’m not putting too fine a point on this by saying that this is not a nice, mellow morning’s worth of fun shooting. It’s a lot of finicky, tedious examination and cleaning, with an occasional shot fired. Each shot is essentially one lapping stroke, one way. I have actually tried to enlarge bores by hand lapping, and after wearing out five laps, and myself, with no measurable enlargement after several hundred strokes, both ways, I do not see how barrels are worn out by any lapping procedure worthy of the name.

    I don’t lap barrels that have shown no problems on the general theory that this will “improve” them even further. It’s too much work, for one thing, and I would assume, in the absence of problems, that the barrel maker knew what he was doing.

    In all cases, bores are shinier and easier to clean. In most cases, there is at least some accuracy improvement. In no case I’ve experienced as there been any further degradation in accuracy.

  14. #14
    Boolit Master
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    Fire lapping can help, but start with a really clean barrel. Valve grinding compound is not good for fire polishing, usually too coarse. MidwayUSA, NECO, Beartooth Bullets - all carry fire lapping materials for firearms. Dave Tubb carries fire lapping 'kits'. A barrel can be too smooth, hard to do with most factory barrels and common fire-lapping kits when instructions are followed. Helps most with barrels that foul badly, giving longer shot strings before accuracy fails. Can also help with rough or very steep throats or tight spots. I have Marlin and Savage 99 barrels that have obvious reamer marks across the lands. They don't disappear, but the do reduce or get less sharp to stop the copper, lead and dirt buildup. Proper fire lapping removes under half a thou (.0005").

  15. #15
    Boolit Master





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    Clover compounds go up to 1200 grit.

    I have a set from Brownells around 20 years old now. Still has another 20 years before I run out they are 1 or 2 ounce each grit.

    Fire lapped or hand lapped this was my choice to lap with.

  16. #16
    Boolit Master

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    If you reline the bore the gun will shoot one hole at 50 yards. Lapping can only do so much. I start lapping with 600 grit and hand lap with a cast plug. Lap from breech to muzzle and finish with oil and rottenstone.

  17. #17
    Boolit Master

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    Valve grinding compound isn't as well graded an abrasive as true lapping compounds. There can be a range of grit sizes, what makes it work for valves is as it is used it breaks down finer and finer finishing better. Fire lapping it goes thru once so no appreciable break down of the grit, this is why the better graded and finer lapping compounds work better for fire lapping. I posted the procedure in mould maitenece on how to make and grade your own lapping compound a couple months ago. Its not hard and can be done.

    Groove dia paper patched bullets will shine a barrel with little metal removal. and may dull the edges of the pits. Here finding the right dia bullet to wrap up to groove size may be the trick. A good paper lightly wiped with light oil will polish a barrel to a nice bright sheen.

    One advantage to firelapping and Paper Patch polish and even hand lapping a bore leaves all the polish lines running in the right direction parallel to the bore not against it. Some of the worst fouling barrels and performing barrels had reamer marks running radially around the bore in places. Throats an leades are bad for this do to the reamer and spinning barrels, cutter lines are rotational.

  18. #18
    Boolit Master
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    Quote Originally Posted by largom View Post
    I have fire lapped dozens of barrels and ALWAYS got an improvement in accuracy! I use JB compound or NECO bore polish grit. 600 grit valve grinding compound might work if you can find it. David Tubb sells a kit with 5 or 6 different grit plated bullets which I have also used with great success. They are available in different calibers and sold by Midway. What ever one uses the key is not to use a course grit.

    Larry
    Valve grinding compound is WAY WAY to coarse for lapping a barrel. You can get much finer lapping or grinding paste on eBay for very cheap...a couple of dollars for enough to do a dozen barrels. They size it by microns here is a handy dandy chart for cross reference micron to grit:
    Grit = Micron = Mesh Conversion Chart

    Grit Mesh Micron
    100,000 0- 0.5 1/4
    60,000 0-1 1/2
    14,000 0-2 1
    13,000 1-2 1.5
    9,000 2-3 2.5
    8,000 2-4 3
    5,000 2-6 4
    4,500 4-6 5
    2,800 5-10 7
    1,800 6-12 9
    1,400 8-20 14
    1,200 10-20 15
    1,050 12-25 18
    800 20-30 25
    600 20-40 30
    500 30-40 35
    325 40-50 45
    285 50-60 55
    240 60-80 70
    225 80-100 90
    160 100-120 110
    100 120-160 150
    AKA hans.pcguy

  19. #19
    Boolit Master Hannibal's Avatar
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    Throat and leade tooling marks are typically the first to get removed just from the simple act of shooting the firearm a few times as that is the end of the barrel that will experience erosion over time.
    Missing the target is not the worst thing you can do.
    Not taking the shot is.

  20. #20
    Boolit Master
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    I agree with JimB. Try larger bullets first. I don't think I have a firearm that shoots groove diameter bullets(cast).

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check