Slugged my GPR Barrel today

[hockeyref]

I slugged the barrel on my GPR project today. First interesting thing is that it is a 5 groove barrel - expected a 6 groove. I measured from land to groove and got .548 with a groove diameter (as the groove is about 2.5x the width of the lands and I can get tis measurement also) of .555...

So.... this has me thinking that maybe the barrel is not a .54 as it's supposed to be.... or is really sloppy QC. :cursing:

I'm betting that it's probably gonna be pretty obstinate when it comes to care and feeding.... Suppose it's deciding on what it's name will be... "Patience"! :shocked2:

So where do you think I should start as far as ball and patch? I was originally thinking about .530 and .535 with pillow ticking or pre cut patches (have some that measure .017 consistently).... Now I'm thinking I may need to go to denim or some other really thick material or a .540 ball....

Or am I way off base on this one?

 

[paulvallandigham]

Unless you are using a barrel micrometer, its next to impossible to measure the diameters on a odd- numbered land and grooves bore. Call Lyman, and ask them about the barrel bore diameter. You can find an 800 number for the company on their website. You can find the website by going up to the member resources section of the Index page, HERE, clicking on " Links" and then looking for the company's name. Then click on the company.

The GPR should be a .54 caliber barrel. You should find good accuracy using a .530" to .535" Diameter ball, with a .015"-.020" thick patch. 80 grains of FFFg powder seems to be the most common powder load used by members here for hunting.

 

[R.M.]

Sounds normal to me.
.540" bore, with .008" grooves, giving you .556" groove diameter.

 

[hockeyref]

Unless you are using a barrel micrometer, its next to impossible to measure the diameters on a odd- numbered land and grooves bore. ........

Paul,
As I stated, the groove is much wider, ~2x wider, than the land so it is feasable to get the groove dia from this. The land measurement is the one that is difficult to get the actual vs calculated measurement.

 

[Trot]

Yeah, it sounds normal to me. Try .530 balls and .018 patches and see how it works, adjust from there if need be.

 

[fyrfyter43]

According to Lyman, the .54 GPR specs are: bore .542" and groove .558".

 

[hockeyref]

Thanks for the input... I'm used to dealing with this stuff in centerfire rifles where you don't really need to figure the bullet diameter and patch combos.

 

[longcruise]

Thanks for the input... I'm used to dealing with this stuff in centerfire rifles where you don't really need to figure the bullet diameter and patch combos.

Try not to get too caught up in overthinking it. Try the .530 and .535 balls with various patch material and see how it goes. If it is still too loose, then get a .540 ball and try that.

While bore dimensions are critcial in the suppository guns due to the precise size of the bullets made for them, we have the flexibility of sizing the ball to the bore.

Finding the combo is just part of the fun of these guns.

 

[BrownBear]

Good advice.

I'll point out that in my experience Lyman barrels start out a little "tight" due to sharpness of the edges on the lands and maybe some machine marks. You are likely to have to use a slightly smaller combo for the first couple of hundred rounds while shooting in or smoothing a new bore, then re-assess.

That was the case in both my 54 cal and 50 cal GPRs. When new the 54 needed .530 balls and .015 patches to seat with anything short of a hammer. That combo started seeming too easy to seat after shooting awhile. I tried .018 patching, and it was back to needing firm seating pressure, and group sizes dropped substantially. After many years of shooting it's pretty easy to seat that combo, and now I'm facing a tradeoff. I could probably improve things again by going to .535/.018, but at the cost of harder seating. It's plenty good enough now and the ease has advantages in the field over and above what would work better on a range.

A similar process is under way with the 50. If I run across some cheap .495 balls I might try them instead of the .490 balls I've been using the last 3 years. But I'm not going to walk very far out of my way to find them.

 

[Dphar]

I slugged the barrel on my GPR project today. First interesting thing is that it is a 5 groove barrel - expected a 6 groove. I measured from land to groove and got .548 with a groove diameter (as the groove is about 2.5x the width of the lands and I can get tis measurement also) of .555...

So.... this has me thinking that maybe the barrel is not a .54 as it's supposed to be.... or is really sloppy QC. :cursing:

I'm betting that it's probably gonna be pretty obstinate when it comes to care and feeding.... Suppose it's deciding on what it's name will be... "Patience"! :shocked2:

So where do you think I should start as far as ball and patch? I was originally thinking about .530 and .535 with pillow ticking or pre cut patches (have some that measure .017 consistently).... Now I'm thinking I may need to go to denim or some other really thick material or a .540 ball....

Or am I way off base on this one?

You really need a plug gage to measure this and this requires an assortment to have the right one.

Take a 3" x 6" piece of patching, lube it and start a ball in the muzzle and see how it fits. The large piece of cloth will allow a "handle" for pulling it back out again.

Dan

 

[hockeyref]

I'll point out that in my experience Lyman barrels start out a little "tight" due to sharpness of the edges on the lands and maybe some machine marks. You are likely to have to use a slightly smaller combo for the first couple of hundred rounds while shooting in or smoothing a new bore, then re-assess.

After I unboxed the kit I degreased the barrel and took a look with a real bright light... Bore was very rough and there were machine\tooling marks on the lands. I lapped the bore with a loose jag, cutting oil, and a scotchbrite - as has been suggested on the board - to smooth out the sharp edges and rough spots (10 strokes, flush and look, repeat for ~50 strokes total). I then followed up with JB & oil on a patch w\tight jag and finally 1 micron diamond polish on a shotgun bore mop. Thought about casting a lead lap but then figured that this is just a factory barrel and I could always replace it with aftermarket if it wanted to be really obstinant.

Like I said, I think she found her name.... "Patience"

Dan,
I like that idea...Will have to get some raw pillow ticking and try it...or intentionally dry ball it! :youcrazy: :shocked2: :shake:

 

[hockeyref]

Just to add one more comment, When I cleaned the oil and shipping crud out of the bore (before lapping) I thought that the lands didn't look too prominent and the grooves didn't look too deep... reminded me of the Marlin "Micro Groove" barrels... I'll do a search through the boad for pics of rifling to compare them to.

 

[Joel/Calgary]

I slugged the barrel on my GPR project today. First interesting thing is that it is a 5 groove barrel - expected a 6 groove. I measured from land to groove and got .548 with a groove diameter (as the groove is about 2.5x the width of the lands and I can get tis measurement also) of .555...

Bore + 2 grooves = .555"; bore + 1 groove = .548". The depth of one groove is then .555"-.548" = .007", so your bore diameter is .548"-.007" = .541". This all presumes exact precision, so you may want to specify .541" ± (2* the precision of each measurement), but it sounds like a .54 to me.

Regards,
Joel

 

[reddog]

Unless you are using a barrel micrometer, its next to impossible to measure the diameters on a odd- numbered land and grooves bore. Call Lyman, and ask them about the barrel bore diameter. You can find an 800 number for the company on their website. You can find the website by going up to the member resources section of the Index page, HERE, clicking on " Links" and then looking for the company's name. Then click on the company.

The GPR should be a .54 caliber barrel. You should find good accuracy using a .530" to .535" Diameter ball, with a .015"-.020" thick patch. 80 grains of FFFg powder seems to be the most common powder load used by members here for hunting.

There is a way to measure a 5 groove slug using a V block,,,but I haven't had enough coffee to remember the formula yet.

 

[BrownBear]

All micro-fiddling and bore lapping aside, the biggest problem I've had with Lyman barrels has been their sharp crown. It can raise cobb with patches and accuracy from a new barrel.

Coning is dead easy and solves the problem completely. I highly recommend it as a standard treatment.

 

[hockeyref]

....
Coning is dead easy and solves the problem completely. I highly recommend it as a standard treatment.

This is definitely ON THE TABLE for the future. Plan to shoot her a little first and see behaves...

 

[necchi]

Coning is dead easy and solves the problem completely. I highly recommend it as a standard treatment.

Boy, there's truth ta that! A friend got a coning tool in .50. After trying it on one, I ended up doing 2 pistols and 2 rifles. That was 5 years ago and they all shoot great! Man!! All ya gotta do is thumb the ball over the patch an stuff'er down, SWEET :thumbsup: . If anything accuracy improved on my sons Traditions Hawkin. I'd kinda like to do it with my 2 .54's, but I'm not really having any trouble and I can't justify the cost for 2 uses .
But my GM is shooting just fine,,don't really wanna fix it if it ain't broke.

 

[ohio ramrod]

Unless you are using a barrel micrometer, its next to impossible to measure the diameters on a odd- numbered land and grooves bore. Call Lyman, and ask them about the barrel bore diameter. You can find an 800 number for the company on their website. You can find the website by going up to the member resources section of the Index page, HERE, clicking on " Links" and then looking for the company's name. Then click on the company.

The GPR should be a .54 caliber barrel. You should find good accuracy using a .530" to .535" Diameter ball, with a .015"-.020" thick patch. 80 grains of FFFg powder seems to be the most common powder load used by members here for hunting.

You can measure the land diameter by simply making a plug gauge. Any one with a lathe can turn one in a few minutes. Just step it a .001 at a step. Groves can be measured with a simple calculation if you have thin enough calipers. Just measure from the land to the flat, then grove to flat. The grove diameter will be twice the difference added to the land diameter. It may take a few tries to find a land and grove that will give a straight measurement but it usually can be done. As a tool and die maker I use pipe mic,s and "anvil " mics myself but the simple plug and calipers will work.

 

[hockeyref]

You can measure the land diameter by simply making a plug gauge. Any one with a lathe can turn one in a few minutes. Just step it a .001 at a step. Groves can be measured with a simple calculation if you have thin enough calipers. Just measure from the land to the flat, then grove to flat. The grove diameter will be twice the difference added to the land diameter. It may take a few tries to find a land and grove that will give a straight measurement but it usually can be done. As a tool and die maker I use pipe mic,s and "anvil " mics myself but the simple plug and calipers will work.

I know where you're coming from.. my dad retired in '86 after 37 years as a journeyman tool and die maker.... when I was a youngster it was great... between the stuff I had access to in an R&D metallurgy lab where I spent 2 years as an intern, and the stuff he could do or have done .....he retired 23 years ago and passed away 13 years ago.. a lot of knowledge passed with him...

As another side note, just ran a 1/16" bit through the flash hole and it was a nice snug fit... Not need to drill her out, but I think I will cone the outside a bit as the screwdriver slot will end up vertical and the inside of the liner is already coned from the factory.

At any rate, this thread is just about to the :dead: stage... Time to finish the metal, put her together, and make smoke, fire, and brimstone. Will post some pics and add a range report once she's had a few loads down her pipe...

 

[paulvallandigham]

If he has access to someone with a lathe, that person is also liable to have the gauges, or tools to measure the bore properly. I have done it several ways myself, but trying to explain how to get an accurate reading those ways is almost impossible without actually showing him how to do it.

I still don't understand why he doesn't call Lyman and ask them for the bore dimensions. Its their barrel, and they should know the specs on it! If he was using a custom made barrel, I would tell him to call the barrel maker. Only if the maker is deceased, or incapacitated, should you have to resort to your own devices to measure the bore and grooves.