Bore Lapping advise/how to

[303Guy]

I have this 1896 Lee Enfield with an SMLE barrel that is over size but has a restriction in the middle of the bore. It's .3075 at the muzzle and the groove is still within the throat and neck dimensions so it'll work but for the bore narrowing in the middle. A .305 plug does not pass through. I've had the bore lapping procedure described to me many years ago but now that I need to do it I would like as much help as I can get from you folks.

Thanks.

[leftiye]

I've never done it, but what I remember says that you might do better with jacketed bullets to remove that restriction. (make sure that the restriction isn't some kind of deposit that needs cleaning first for sure) Jacketed will size down less before and during hitting the restriction. Use something like Wheeler's bore lapping abrasives. There's something like 200 grit, 400 grit and 600 grit. Use a few of the coarser loads, then finer and finer. Put the grit on a steel plate and roll the bullet between two plates. Load slow velocity loads with fast powders that won't squib on you (make sure they exit the barrel each time). Just enough to completely transit the barrel. Abrasives become more aggressive with both pressure and speed.

[303Guy]

I've been wondering about fire-lapping but am not sure how suited it is for opening up the centre of the bore. I have fire-lapped the bore but not aggressively, only enough to remove rust and crud. Strangely, the muzzle area did not brighten up. Small wonder since the middle is tighter. Finding suitably sized jacket-eds might be a problem too unless I can swage up or size down something available. I need to remove material from the grooves at the same time. The groove diameter is something like .316. Of course, just because the bore restricts in the middle does not mean the grooves restrict. Perhaps I should measure that first.

[Jim Flinchbaugh]

The muzzle didn't brighten up because your bullets got squished going through the tight spot :)
I'm about to hand lap my Redhawk, I was advised to find a smaller diameter bronze bore brush, plug the barrel, insert the brush and pour melted lead in to the brush, Expose part of the freshly poured lap, apply compound and have at it. When I get brave enough to try it, I'll post the results.[smilie=1:

[725]

I've firelapped by using a grit compound imbedded on the bearing surface of FMJ ammo. Rolled the ammo over the steel plates covered in grit. Shot a couple, cleaned and examined. Shot a couple, cleaned and examined, and so forth until I reached my objective. Worked like a champ!

[tomme boy]

I was always told to use a lead boolit if you have them to fire lap. Also to load the rounds as slow as possible. You want the lead to just make it out the barrel.

[longbow]

I would cast a slug and hand lap at least to remove the tight spot. I have done a .303 and 1894 Marlin with no issues and significant improvement.

I used a cast boolit drilled to fit over the reduced end of some 1/4" round bar and retained with a nut turned to clear the lands so basically a threaded ring.

- boolit was persuaded to fit the bore with a mallet and run through with some oil
- now engraved boolit was chucked carefully in a lathe and drilled
- drilled boolit was fitted to rod and retained by "nut"
- lube grooves filled with valve lapping compound (425 grit I believe ~ a bit coarse but it worked well)
- muzzle protector used to prevent rod from touching the barrel
- run grit impregnated slug through the barrel back and forth to lap
- replace grit as required to keep cutting (you can feel when it needs more)
- replace lapping boolit as required to keep cutting (again, a "feel" thing)
- concentrate lapping at the tight spot(s)
- I also tended to spend more time lapping the breech end of the barrel than muzzle to produce a very slight "choke"
- go slow and easy
- check often
- clean ALL the grit out of barrel, action and anything else you care about that was touched or in the vicinity of lapping compound ~ it gets everywhere.

Alternately you could cast a lapping slug around a rod placed in the barrel with a plug to stop molten lead to make a boolit length slug. I think this is the "standard" method but I figured a swaged boolit would produce sharper edges than cast so went that route. Also, casting into a .303 bore and around a rod seemed like more trouble than I wanted.

The Marlin is microgroove so I was hesitant as there just isn't much rifling but accuracy was poor, it had the dreaded Marlin "tight spots" under dovetails and it leaded so I decided to give it a go.

I worked mostly on the tight spots with an occasional full length stroke then once tight spots were gone I worked several short to medium strokes at the breech end then a full length stroke or two and repeated until feel told me there was a slight taper. Now there is no leading and much improved accuracy... and the microgroove rifling doesn't show any signs of being removed. Overall, the barrel is smoother and shinier than when new.

For the .303, the exercise was mostly to clean it up and again, it worked well.

FWIW

Longbow

[303Guy]

Thank you all. I think I'll try the swaged slug first and concentrate on the tight spot. The bore seems to actually taper down to that spot. I'll probably follow that with fire-lapping. The throat is too tight too so it should only be of benefit.

I've mentioned that the restricted area is less than .315 with a muzzle of .3175. That's a lot of material to remove it seems.

[Multigunner]

SMLE MkI bores were reverse taper lapped from around midway of the bore.
The purpose being to reduce bullet friction so the old MkVI ammo would give the same velocity from the short rifle barrel as from the older LE long rifle barrel.
This actually worked but accuracy suffered, and variations in the lapping process often resulted in greatly oversized bores at the muzzle.

Reverse taper lapping, coupled with excessive wear of the first ten inches of bore from firing the MkVI ammo loaded with the old Cordite MkI propellent could give the same effect as a bore restriction at the mid point.

A worn leade with throat appearing to be too tight could be due to rapid build up of jacket fouling in the throat that partly protected the throat.
Hiram Maxim's experiments with Cordite in his early machinegun barrels showed that more erosion took place from around three inches up to ten inches into the leade than in the throat.
Cordite Mk1 burned at just short of 5,000 degrees F.
Publications of the day state that almost the entire charge would be burned in the first nine inches of bullet travel, though expansion of gases continued to the muzzle. Temperatures and heat transfer to the bore surface peaked in the first nine-ten inches so erosion from that point on was greatly reduced.

Australia continued to use the MkVI ammunition throughout WW2 and for many years afterwards.

[303Guy]

Thanks for that, Multigunner. I've been having a close look at the muzzle area and it's quite rough. So, this would seem to be a candidate for my new pig gun. I've determined that the bore opens up from 350mm onwards. That would give a me a true bore length of 300mm (1 foot to my American friends). It now seems pointless lapping out the constriction if the rest of the bore is that rough. (I really wanted a long and light barrelled open sighted gun).

Maybe I should simply use my aggressive bore-lapping trick and gain a larger throat with a bore that tapers down to the muzzle which would of course be moved back slightly. (That short barrelled pig gun of mine is so sweet to carry around). So my plan would be to remove the scope and mounts from my original pig gun and fit shotgun type 'sights' and scope this one, making an ideal two-hunter pair of pig guns.

This is it. The second to the rear groove holds the lapping compound within the neck.
http://i388.photobucket.com/albums/oo327/303Guy/MVC-565F-1.jpg
It gets into the deepest rust pits and corners and cleans out a bore in just one or two shots. It has a downside - it can jamb in the throat then it's fun and games getting it out. Especially if the gasses can't escape. My new ones will have venting grooves in them. I won't mention what happened when I opened the bolt on the last one that jammed. It wasn't that bad but lesson learned. Make vent grooves and use enough powder!

The front bit is bore size to old the thing straight and the next ones are to align the rest of it as it enters the throat.

After firing.
http://i388.photobucket.com/albums/oo327/303Guy/MVC-107F.jpg

[303Guy]

Right. Did a little fire-lapping. First with a paper patched boolit then with my fancy aggressive brass fire-lapping slugs. I'm doing a new thread on this so if you all want to have a look ....

[leftiye]

Size down 8mms?

[303Guy]

I'm thinking about that. I might just make a number of my 'fire-lapping slugs' and see what happens. It's time versus money and right now I'm broke. :( I suspect the 'fire-lapping slugs' might work better because of the amount of grit they hold.

[303Guy]

It sounds like fire-lapping with soft lead boolits will do the trick. I might just go right into it.

[daddyseal]

I have this 1896 Lee Enfield with an SMLE barrel that is over size but has a restriction in the middle of the bore. It's .3075 at the muzzle and the groove is still within the throat and neck dimensions so it'll work but for the bore narrowing in the middle. A .305 plug does not pass through. I've had the bore lapping procedure described to me many years ago but now that I need to do it I would like as much help as I can get from you folks.

Thanks.

I would be very light on lapping that bore, friend.
That particular rifle bore was made to narrow as bullet goes thru...narrowest at muzzle.
The makers said it improved accuracy and velocity, as It was made for snipers of the day for Australia and Canada.

[daddyseal]

SMLE MkI bores were reverse taper lapped from around midway of the bore.
The purpose being to reduce bullet friction so the old MkVI ammo would give the same velocity from the short rifle barrel as from the older LE long rifle barrel.
This actually worked but accuracy suffered, and variations in the lapping process often resulted in greatly oversized bores at the muzzle.

Reverse taper lapping, coupled with excessive wear of the first ten inches of bore from firing the MkVI ammo loaded with the old Cordite MkI propellent could give the same effect as a bore restriction at the mid point.

A worn leade with throat appearing to be too tight could be due to rapid build up of jacket fouling in the throat that partly protected the throat.
Hiram Maxim's experiments with Cordite in his early machinegun barrels showed that more erosion took place from around three inches up to ten inches into the leade than in the throat.
Cordite Mk1 burned at just short of 5,000 degrees F.
Publications of the day state that almost the entire charge would be burned in the first nine inches of bullet travel, though expansion of gases continued to the muzzle. Temperatures and heat transfer to the bore surface peaked in the first nine-ten inches so erosion from that point on was greatly reduced.

Australia continued to use the MkVI ammunition throughout WW2 and for many years afterwards.

Exactly, lapping can remove the designed taper towards the muzzle on those sniper rifles of the day~!!

[nanuk]

if the reverse taper means "Wasp Waisted" then the bore was NOT narrowest at the muzzle.

as 303Guy has measured, HIS is wasp waisted, so he could go hard on lapping an not affect the muzzle much at all


as an aside, when lapping using a poured lap, it has been suggested that after charging with grit, one would swage the lap in the bore with a brass rod with the lap down at least 6 inches.

this protects the muzzle from initial damage also

[leftiye]

Nanuk has the easiest way to get a lap that fits. I'd swage it just outside the tight spot. Though just forcing it to the tight spot from the muzzle end (should be smaller than the rear portion?) would do the same thing. If it's tapered towards the muzzle it should let loose right after being started, and possibly get tight again at the restriction. If it doesn't get tight, then swage it in place.

You could lap only the restriction by driving the abrasive embedded bullet back and forth at the restriction. You could periodically enlarge it by impact swaging it just outside the tight area.