Indexing new barrels via breech thimble? What vendor?

[PCPhack]

Has anyone set up a breech with a thimble of sorts, so that a barrel could then be rotated in it for indexing purposes?

I am curious if anyone knows of a gunsmith doing something like this for air rifles. I want to purchase 1 or more barrels, but also want them indexed before machining a port for a TP.

Alternatively, a fancy enough breech that attached the barrel just after the port could be indexed by an end user themselves.

[mattymatt90]

Rapid Air Weapons.. they have a rifle that can be set up like that.

[nervoustrigger]

The simplest arrangement I’ve seen that allows the user to index it on the fly is having a series of ports drilled around the circumference of the barrel, along with a groove connecting all the holes which is lined up with the valve’s exhaust port.
 
There is a slight downside to it, that the porting represents an additional volume downstream of the valve which wastes a bit of air.  However once you get it indexed where you want it, you can simply use JB Weld to fill in the groove and all but the one port at the 6 o’clock position which lines up with the valve.
 
Regarding the approach which attaches the barrel just beyond the port, I vaguely recall seeing an example of that.  If you plan to go that route, one consideration to keep in mind is that the bore is seldom perfectly concentric to the OD of the barrel, so it may not be trivial to avoid a step or discontinuity between the loading ramp and the barrel.  Ideally you want the pellet seated by way of a perfectly smooth leade so as to avoid any potential of damage to the head (or to a somewhat lesser extent, the skirt).  The perimeter of the head is critically important to accuracy, and unfortunately rather easy to damage both because of its geometry and the softness of lead.

[rsterne]

I did one gun with a thimble so that I could index the barrel.... It was such a PITA I haven't done another....



Thimble in place, threaded retaining sleeve slid forward on barrel.... The collar behind the retainer is threaded and loctited in place, it takes the end thrust of the retainer, as the barrel is in tension from the muzzle....



Concentricity and alignment are very difficult to achieve.... I have not done sufficient testing to see if indexing the barrel makes any difference in accuracy....

Bob

[PCPhack]

Very cool Bob. I was wondering if you had done this.

It sounds like there are probably not any vendors readily available that will perform both barrel machining and initial indexing.

[Motorhead]

Thimbles correctly done can be useful, but also can create there own side effect issues, such as the following ...

The gap between barrel and thimble will leak air during the shot. It can be o-ring sealed at the slip fit connector or X2 on barrel/thimble O.D. to receiver.
T.I.R. ( total indicated run out ) needs to be very close between barrels bore & thimble bore so no snagging of pellet happens as it passes by the intersect.
Thimble end at breech generally will require a dimple where the retention set screw holds it, or during shot cycle the it will be blown rearward. ( Set into a shoulder if practical another solution ) 
Thimbles are most stable within LONG receivers where the support in length LOST to the thimble is still generous enough to hold barrel stable & securely.
Pending the rifles Breech and if pellets flush seat, deep seat via probe the WHERE pellet rests prior to be shot can have it IN THE THIMBLE or In the barrel past thimble ... the throat diameter within thimble must address this and TIR mentioned above becomes most important especially when pellet is held in the thimble and fired into the rifling.

Anywho ... just some thoughts on the subject. Not a negative but stuff one should think about.
Personally find that using adjustable Scope mounts is a lot easier if you have to chase a barrel bore out of parallel to receiver / mounting.

Your mileage may differ 

Scott

[PCPhack]

Thanks Scott. I figured that someone had made a breech/receiver just for indexing barrels. After the index point is made, the barrel could then be traditionally machined. Wouldn't such a tool be useful to someone that machines and sells finished barrels?

[Motorhead]

Thanks Scott. I figured that someone had made a breech/receiver just for indexing barrels. After the index point is made, the barrel could then be traditionally machined. Wouldn't such a tool be useful to someone that machines and sells finished barrels?

Simpler way if wanting to pre-index NOT being able to shoot it ...
Place receiver into a padded vice, have barrel machined to fit receiver, tho no transfer port or thimble machining done.
have barrel / receiver set up in such a way you can BORE SIGHT threw it and able to see 100 yards or more while viewing threw it.
Place a target, picture or some point of reference you can focus your bare eye upon while turning the barrel looking down the bore.
ROTATE barrel and note what your downrange view is doing for up,down,left,right CLOCKING the barrel to where you want POI to be. ( could be a optically centered scope already mounted, parallel to receiver or some place you want it clocked too.  Once you view threw barrel is where you want it clocked .. MARK IT.

OPTION 2 ....
Mount barrel within receiver and using a few pointer / near contact points fingers near muzzle, rotate barrel and note the out of round wobble and clock barrel either the typical 12 or 6 o'clock or anywhere else felt required.

Remove barrel and finish machine work Thimble or Not

[PCPhack]

Nice. That sounds much easier.

[tkerrigan]

I know you can index the barrel on a p12 Spa as it comes, I haven't had my p15 apart yet to see.  Regards, Tom