Chambering Prebore Methods

[jackieschmidt]

The pictures are not in sequence.

I rough out the tenon OD, drill to remove excess stock, then reach in with my last word indicator and indicate the throat area. I carefully watch the indicator as the stylus passes from land to groove. I then single point bore the chamber, using the compound set on the same taper as the reamer. I then establish the best chamber I can in the barrel, reaming by hand with the center supporting the reamer.

I then finish the OD of the tenon, face the shoulder for the proper headspace, then thread the tenon. I then machine the cone using the compound set on 29 degrees. Polish and install.

 

[jackieschmidt]

Before everyone throws their hat into indicating the lands, ask yourself this. Looking at a cross section of a typical 4 groove barrel, what percentage of the bullet will ride on the tops of the lands, or ride the grooves? The bullet will conform to the barrel interior, the majority of which are grooves. The lands twist it, but the bullet is influenced by the grooves. If you pay real close attention while turning a barrel, you will see all kinds of "differences" from one groove/land to another, in some barrels. It's my belief, since we are dealing with the breach end, that lapping can/may create some of this. I believe Tony's book says he wants his barrels indicated on grooves, not lands. Been a while since I read it, but this has been known long before his book.

People who indicate with range rods have no idea about the condition of the lands and grooves.

I have found with Krieger and Bartlien Cut Rifled barrels, the four lands and grooves are so close to being equal that my Mitytoyo or InterRapid .0001 indicators will not pick it up as the stylus goes from one to the next.

in all of my years of chambering, I have only had one barrel that had a shallow land. I sent it back, and the manufacturer agreed and sent me a new blank.

'

 

[Judd]

Thanks everyone for sharing. I’m still learning LOTS and I love these posts to see the different ways guys are skinning these cats. I am under 10 barrels still and thus far everything has shot from surprisingly excellent to plenty good enough. I’m still using the reamer the whole way but I’ve been thinking this is my next progression. I need to figure out the prebore.

So far, the best secret I’ve stumbled upon is how well those Wilson button barrels shoot and they are 1/2 price a normal cut barrel I would buy. First two, I did were those and I’ve been shocked at how well they have shot.

 

[Tucker65]

Jackie,

I see you are grabbing the barrel in a 3 jaw chuck. It appears to be an adjust-true style of chuck. Is that your usual method of dialing in the bore? I use a 4 jaw chuck which seems like it would be a little easier to dial the bore in with instead of what appears to be 3 jacking screws to adjust the chuck on your lathe. But maybe I'm not seeing everything in your pictures.

 

[tobnpr]

I’m still using the reamer the whole way but I’ve been thinking this is my next progression.

Pre-boring isn't just about reamer wear, if it were- I probably wouldn't bother. With a flush system the process is plenty fast enough for me to make $$- and the cost of the reamer itself allocated over the number of chambers that can be cut with it is mostly insignificant to me in the overall picture.

What pre-boring does is establish a hole that's perfectly concentric to the spindle axis. If you're indicating a couple of inches or beyond the breech, that location may or may not be perfectly in line with the bore at the breech end of the barrel. By boring the "entry hole" after you've dialed in the throat you ensure any misalignment is removed. With match-grade barrels, I think this is largely an exercise of "just to be sure"- they're mostly air-gauged to within a couple of tenths tolerances so there shouldn't be any difference that would translate at the target.

All depends on which two points you indicate to establish your center. Some indicate the muzzle, and the throat, some neck and throat. Whatever two points you work from, boring guarantees the hole at the breech is in line with them.

 

[lb-ft]

There is no limit to how much can be learned as a machinist.

 

[jackieschmidt]

Jackie,

I see you are grabbing the barrel in a 3 jaw chuck. It appears to be an adjust-true style of chuck. Is that your usual method of dialing in the bore? I use a 4 jaw chuck which seems like it would be a little easier to dial the bore in with instead of what appears to be 3 jacking screws to adjust the chuck on your lathe. But maybe I'm not seeing everything in your pictures.

My SetTrue 3 jaw has 4 adjustment screws, Most do.

It makes dining in 4 groove barrels easy. 5 grooves is a tad more challenge.

In my opinion as a Machinist, who ever invented the SetTrue chuck deserves a special place in heaven.

I shot these three 5-shot groups today With the barrel in the pictures.I think it’s ready for the TackDriver.

 

[Bob3700]

People who indicate with range rods have no idea about the condition of the lands and grooves.

I have found with Krieger and Bartlien Cut Rifled barrels, the four lands and grooves are so close to being equal that my Mitytoyo or InterRapid .0001 indicators will not pick it up as the stylus goes from one to the next.

in all of my years of chambering, I have only had one barrel that had a shallow land. I sent it back, and the manufacturer agreed and sent me a new blank.

Jackie,
If the Krueger n Bartline bbls lands n grooves are so close, then how is a range rod a hinderance in dialing in the throat. Just asking?

 

[jackieschmidt]

Jackie,
If the Krueger n Bartline bbls lands n grooves are so close, then how is a range rod a hinderance in dialing in the throat. Just asking?

I do not see it as a hindrance at all.

I feel safe in saying that premium barrel manufactures such as Krieger and Bartlien do not let barrels that are not up to their standards get out the door.

 

[ar15Jeff]

First of all, I am not a machinist or gunsmith, but would like to as a hobby. I am gathering information on how to thread and chamber barrels. Question, how does indicating a barrel using the groves more accurate if the pilots on the reamers follow the top of the lands? The top of the lands are the original bore hole and the grooves are all indexed from them when the barrel maker cuts them (cut rifled). To me, as a nubie, it wouldn't make a difference. I am basically looking for the easiest and most accurate procedure that has least chance to screw up for beginners like me. Just asking.
Jeff