JB Bore Paste for polishing?

[jmerchant.1]

Has anyone used JB Bore Paste specifically for removing tool/machine marks to smooth and polish a chamber and bore? Did it improve accuracy? For strictly cleaning purposes I’m quite happy with Wipe-Out, but the chamber on one of my factory rifles looks pretty rough from the reamer and isn’t very accurate. Thought maybe a mild abrasive would help? Here’s a borescope pic of the throat I’m referring to

 

[HighUintas]

Yes. I know some people will use jbs or something very similar on every factory rifle they buy, but it takes a LOT of strokes and refreshes on patches to smooth it out.

 

[Wapiti1]

JB's isn't aggressive enough for that throat, or to really remove any metal at all. Fire lapping will be the easiest option for that throat. For just the throat, I'd run 3 rounds of each of the second to finest and finest grits and check the result. This is a good application for that process.

If you really want it pretty, rent a throat reamer and extend it 0.005" to clean it up. Or just, shoot it and see if it settles in. I suspect it will start to come in after 50 rounds and tighten up from there.

Jeremy

 

[4and1]

Jeremy is right, JB will not remove metal. Factory barrels can be very rough. I haven't done much fire lapping cause it moves the throat forward, and most factory chambers already have generous throats.

Most think a custom high grade barrel is smooth, but when those are lapped, they generally use compound in the 150 grit range. Polish a barre; mirror smooth and it will copper foul like crazy.

 

[jmerchant.1]

Polish a barre; mirror smooth and it will copper foul like crazy.

How can this be? Wouldn’t a rougher surface grab the bullet jacket and collect copper more than a mirror smooth surface? The whole idea of breaking in a barrel and lapping is to smooth imperfections away

 

[rayporter]

gunsmiths say you dont want a mirror smooth chamber. that it causes poor extraction. many even put cross hatches in the chambers they cut for this reason.

i am just repeating what i have heard.

as to fire lapping, you can read just about anything and choose what you wish to believe. by and large top smiths are against it.

if you want to test a paste cleaner do it on a piece of barrel or under the barrel where it wont be seen and go to town with a drill and some compound. report back in a week [nonstop] if you have removed any metal.

 

[HighUintas]

I think it was something like 1000 strokes with JB that it took to smooth anything out. I've not tried that, but that's what a guy told me he did for all his rifles and it helped. So, take that with a grain of salt I guess.

The way I do it, and I actually just did this to a very nice barrel with a crappy throat reaming job, is to use 3m pads. The different color pads have different grit levels and you have to be careful. I'll take a maroon color 3m pad, a bronzer brush that is the next lower caliber (say a .284 brush for a .308 bore), cut a strip of the pad to fit on the whole brush perfectly when wrapping it around the brush, delaminate the pad into two pieces so that it's half as thick, then wrap the pad on the brush.

As you work the throat with the pad, it will break down and increase in fineness. After you're done, you use JBs to smooth the surface of that finish (or close the pores).

The detailed instructions can be found in Nathan Foster's guide to bolt action accurizing and maintenance. I wouldn't do it without reading this section in the book because you could screw it up. I've done it and it helps.

 

[4and1]

How can this be? Wouldn’t a rougher surface grab the bullet jacket and collect copper more than a mirror smooth surface? The whole idea of breaking in a barrel and lapping is to smooth imperfections away

Speaking specifically about a new barrel with the chamber cut by a reamer, if a barrel is lapped to final finish, the lapping strokes are in the same direction of bullet travel. The "breakin" procedure is more intended to smooth out the throat machine/reamer marks than the barrel itself. Turning a barrel in a lathe to chamber, which all are done, leaves the cutting marks perpendicular to bullet travel, and sometimes these marks are deep. Breakin is a good thing.