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The Hog Whisperer (Administrator)
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Discussion Starter #1
Anybody ever see odd velocity readings after lapping a bore? That is, loads that had been pretty consistent, now 100+ fps faster, then back to the usual, then back up again?

First outing with a Glenfield .30-30 after lapping what turned out to be a very tight bore. My chrono was reading very consistently with the previous rifle loads so I don't think it was the issue.....

Just very odd. Maybe should have broke it in with some jacketed bullets first?
 

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Wondering about the reloads now. Pure handloader speculation. Throat was changed due to fire-lapping. Nice and clean, right?

Partial neck-size the same cases and see what happens?

Cheezywan
 

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The Hog Whisperer (Administrator)
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Discussion Starter #3
Good question. I loaded the same OAL as before, but did not check to see if the throat had moved forward.... thanks!

My method of resizing is to only push the case shoulder back 0.002" or so. Ensures everything goes in the chamber without overworking the brass.
 

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A Hunnard Huh ??

If the standard deviation of your reloaded ammo is 100+fps I would say it is because of something other than simply lapping the bore. I have lapped quite a few weapons including ML's,BUT......I have never shot them over a chrony,before and after.

Also....I have never though about the throat moving forward unless you lapped with the aid if a pneumatic grinder.(PLEASE....say you didn't) If the "vigorous" lapping did(?) set the throat forward I suppose it could be by the distance of a gazillionth of a inch. Basically,not enough to matter.

I have never heard of a proper bbl lapping doing any harm to any bbl. I have done revo's with the aid of a Dremmel tool,both the cylinder and forcing cone and never seen negative effects.

I'd say check your load data,mic a few of the bullets used,mic some of the brass used,check for overly deep or shallow primer installation,also OAL of the brass used. If all within specs,I'd be interested in the "finding" of the 100+fps SD. -----pruhdlr
 

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SWAG: Try reloading with new brass and run the bullets through your chronograph again. Guessing inconsistent neck tension with mixed/used brass?
 

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If the standard deviation of your reloaded ammo is 100+fps I would say it is because of something other than simply lapping the bore. I have lapped quite a few weapons including ML's,BUT......I have never shot them over a chrony,before and after.

Also....I have never though about the throat moving forward unless you lapped with the aid if a pneumatic grinder.(PLEASE....say you didn't) If the "vigorous" lapping did(?) set the throat forward I suppose it could be by the distance of a gazillionth of a inch. Basically,not enough to matter.

I have never heard of a proper bbl lapping doing any harm to any bbl. I have done revo's with the aid of a Dremmel tool,both the cylinder and forcing cone and never seen negative effects.

I'd say check your load data,mic a few of the bullets used,mic some of the brass used,check for overly deep or shallow primer installation,also OAL of the brass used. If all within specs,I'd be interested in the "finding" of the 100+fps SD. -----pruhdlr


Lapping with a Dremel tool? Polishing maybe, but not lapping
 

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The Hog Whisperer (Administrator)
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Discussion Starter #7
Good thoughts. More info: brass on 2nd loading, I have chrono'd loads with this brass (on first firing) and same lots of powder, bullets, and primers. Neck tension should be the same, I am using the same resizing die, and also, the same expander (a Lyman "M" die modified for that bullet diameter). Primer pockets are cleaned with a uniforming tool, so they darn sure ought to be the same! Cases are primed with a hand tool and should be the same as it bottoms out at a couple thousands below the case head.

Lapping done with some original LBT compound that I have a tiny bit left of. 18 or 19 rounds, 2.5 grains of Bullseye with the rest of the case full of polyester pillow stuffing. Basic low velocity lapping load.....

Yeah.... confusing as heck! I simply do not understand it.
 

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The original bullets, after lapping, may be undersized for the new bore diameter. Slug your lapped bore if you havent allready, and get some bullets that are .002 over the lapped bore diameter.....then, work up a new load (start a bit low on the powder charge)...same powder, primers, cases, etc, with the new bullets................also, I do not put a lot of faith in ES/SD info. I have had some competition Benchrest loads shoot tiny groups with ES/SD's all out of wack, and, I have had some out of wack groups show low ES/SD's. If the velocity is where you need it to be, and your shooting tiny groups, just disregard the ES/SD readings!
 

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The throats in the Garand barrels I rebuilt for our club and lapped sixteen years ago moved forward only about 0.0010", according to the military throat wear gauge, and this is after a pretty rigorous lapping job. It was done using a NECO kit. The Garand military barrels tended to have a significant constriction below the lower band of half a thousandth or so, and that took awhile to remove. It took about 20 rounds of 240 grit, 10 of 400 grit, 10 of 800 grit, and 20 of 1200 grit to complete. but still didn't move the throat much.

I also still have some 20 year-old LBT abrasive. It seems to be garnet and is finer than the lab grade 240 grit coarse abrasive in the NECO kit, so I doubt you've changed the throat dimensions much.

Reduced start pressure, though, is another matter. No doubt the bullets are sliding in more easily, and that can be all it takes to make ignition erratic. Try magnum primers. Try a crimp increase. Try seating out near the lands. Try seating deeper. Try faster powder.

Be sure you aren't distracted while priming (I tend to be guilty of this, doing it in front of the TV). A slightly underseated primer can make that kind of velocity difference, too. The author of one of the chapters in the Precision Shooting Reloading Guide said he is able to get most rifle loads down to 10 fps deviation by seating primers hard; beyond mere anvil contact.
 

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The Hog Whisperer (Administrator)
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Discussion Starter #10
I forgot to mention that the ammo was crimped with a Lee Factory Crimp die. So.... not sure it can get crimped any more. If I wasn't already using one, then I'd take that step.

Bullet seating depth was originally set up so that the rifling left marks all the way around the bullet when chambered. I need to ensure that it would still do this, but seems unlikely the throat went too far forward.

Size.... it slugged out just under 0.308" - unusually tight for a Marlin. Cast bullets are sized 0.310" so should be OK. I have not re-slugged it yet though.

Anyway, I wouldn't normally get too wrapped up in extreme spreads, but thought it was quite unusual to see this after lapping when it wasn't doing it before.

Mystery to me......
 

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How about the old distance from the chronograph bugaboo?
 

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Mike; I normally gain about .002 (on average) in bore diameter when I lap a bore...either by hand, or by fire lapping. The last two rifles I did were fire lapped using Marshalls kit, and following his directions. They were both stainless Marlins, and had the usual bore contsrictions. If you were 308 before you lapped the bore, and you fired maybe 20 lapping rounds or so, you would have probably gained about .002 to .0025 or thereabouts...thats what I have experienced. I think that you should slug the bore, and get the dimension as it stands now, just to make sure those bullets are not undersized....When I fire lap, I usualy slug the bore after the first 10 rounds, and then every 5 rounds after that to keep an eye on how much is being gained. I slug the bore before I start...know how much needs to be removed, and only firelap until I reach that dimension. then I polish the bore.....................regardless of all that, and regardless of the ES/SD numbers, does the load shoot accurately?
 

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I'd be tempted to not push the shoulder back those "0.002" that you mentioned. Just to see what happens? Size the case only far enough "grab" the bullet at your seating depth.

May or may not chamber hard that way? Second shoulder to align the bullet with the bore can be a good thing sometimes.

Eliminates some percentage of brass difference from the "let go" factor.

Good luck!

Cheezywan
 

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Hey Mike. Sorry to get in on this late but I had nothing to add. Still don't. Just questions and waiting to see some answers. So just know I am watching and if anything comes to mind from my limited fire lapping experience, I will let ya know.

What did the bore need, and did the lapping improve or do what you wanted?
 

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The Hog Whisperer (Administrator)
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Discussion Starter #15
I haven't slugged it again yet, but need to. Noticed tonight that there are some lead streaks near the muzzle. Wondering if it needs more lapping?

I need to sort through the chrono data, possibly retest. I suppose the chrono could have been acting up as I did not use it afterwards with a known load.....

Gonna run some factory ammo through it too.
 

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Do a muzzle slug and a throat slug and measure with an OD thimble micrometer so you can resolve ten thousandths. The muzzle streaks may mean nothing more than a taper toward the muzzle? Last time I did a rifle it seems to me the throat end of the bore opened up half a thousandth and the muzzle about one and a half ten thousandths. It was enough to remove all rough toolmarks at the muzzle, but no more.

Per an old Precision Shooting article's instructions, I use an 8X loupe and watch the rifling next to the muzzle crown until I just see the abrasive marks clean off the original surface texture. If there are very slight depressions in the lands from the tooling, I ignore them as long as they aren't sharp.

I also reslug the full length of the bore every few rounds to check progress on any bore constrictions. My old NECO kit's lab grade abrasives cut very fast, so it's every five rounds with that kit's 240 grit starting grade, but I think Marshall's 320 grit Clover compound should go twice that many between slugging checks. Constrictions can make me fire beyond the muzzle tool mark clean-off point, but muzzle clean-off sets the minimum.
 

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The Hog Whisperer (Administrator)
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Discussion Starter #17
I went back and reviewed the data. There was actually a fairly orderly progression, considering it was only 3 shots per powder charge increment (per the OCW method).

There were some examples where bumping up .3 or .4 grains of powder ended up with a slower load, but in the end the averages always went up. Guess the overlap was just more obvious when shooting them in the round robin sequence, and recording each velocity.

Think the barrel needs more lapping .... sorry for the false alarm!
 

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The Hog Whisperer (Administrator)
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Discussion Starter #19
Because the gun was lightly speckled with rust, and it was a devil of a job to drive a slug through the bore......
 
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