6mm TallDog

[mtngun]

The 6mm BR project came to an end when I discovered that it liked to throw gas checks at the chronograph. I never did determine why it hated gas checks other than to say that the problem was greatly reduced, but not eliminated, after I chamfered the entrance to the throat. Apparently the checks had been "catching" at the entrance to the throat?

Anyway, since it wasn't obvious what was wrong with the BR chamber to cause it to hate gas checks, I elected to set the barrel back and rechamber. And as long as I was rechambering, why not try a 6PPC, since the BR had too much powder capacity for my cast loads? Well, because the Remmy has a 308 bolt head, that's why, and there are safety issues with some of the custom bolts that use an M16 or Sako extractor.

I considered converting a bolt to the Savage bolt head system. The Savage gas baffle would address the safety issues, and a PPC bolt head is available for the Savage. Only problem is the conversion costs money.

How to get PPC case capacity with a 308 bolt head? Well, I am not the first person to ask that question. Decades ago, match-grade PPC brass was scarce so benchrest shooters attempted to duplicate PPC ballistics by shortening a BR case. It was called the 6mm Talldog (sometimes spelled Taldog). It was a pain to make the Taldog cases so when PPC brass became readily available the Talldog was largely forgotten.

Nonetheless the Talldog would seem to meet my requirements, so let's give it a try. Here's the two cases for comparison. The unfired Talldog case has wrinkles at the neck from the forming operation -- I'm hoping the wrinkles will blow out when the case is fired, otherwise I will be forced to rethink my case-forming procedures.


No, the wrinkles did not blow out. So back to the drawing board ....... I added another step, as shown. I had to shorten a few dies to Talldog length, but once that was done and the process figured out, case forming went quickly.


My TallDog chamber was cut with the same reamers as my previous BR chamber:
-- 0.270" neck (tapers to 0.269")
-- generous chamfer at entrance to throat
-- freebore starts out at 0.246" (the freebore has a slight taper)
-- 1.5 degree per side leade
-- the throat doesn't hit 0.242" until 0.200" past the case, so it's a fairly generous throat.
-- 1.415" max case length (0.140" shorter than the BR).
-- 26 3/8" barrel.
-- a fired case holds 33.5 gr. H2O, vs. 32 - 33 gr. for a PPC case (according to Quickload, anyway).

[mtngun]

First range session with the Talldog. Today's goal was to fireform cases, verify basic functionality, and verify that gas checks are staying on the bullet. I didn't have time to cast & prepare bullets for this chamber, so instead I used an assortment of "leftover" bullets from previous experiments with the 6x45 and the 6BR. Instead of using my preferred powders, I used a 30+ year old can of IMR 4198, just because it needed something to do. For the most part I did not bother to measure the jam point for the assorted bullets, instead I just arbitrarily chose a seating depth. I did not expect these "leftover" bullets and randomly assembled loads to set a new world record.

First off, I fired 10 gas check loads, point shooting, 15 feet away from the target, to see if the gas checks were hanging on. The 10 shots left 10 holes with no "extra" holes. The rest of the shooting was done over a chronograph and no errant checks hit the chronograph, so I'm going to declare a victory on the 6BR's errant gas check problem. For the time being I give credit to the Talldog's sloppy throat that allows the bullet to enter without "catching" at the entrance of the throat.

The cute little Talldog.


Even considering that the 4198 powder was faster than ideal for the velocities, and even considering that the "leftover" bullets and random loads were far from optimal, today's target was disappointing. The bore rider was the worst of the worst while the 80 gr. Loverin was the best of the worst, suggesting to my way of thinking that the accuracy problem was due to a poor fit in the Talldog's long sloppy throat.


Conclusions and Observations:
-- the new chamber and sloppy throat solved the errant gas check problem.
-- still, that doesn't explain why of all my barrels, only the 6BR tossed its gas checks?
-- the long, sloppy throat does not appear to be accurate (so far, anyway). This is hardly surprising.
-- the nose of the 75 gr. bore rider was grazing the lands when chambered, but not engraving enough to my way of thinking.
-- after firing the uncoated bore riders, there was slight leading in the throat. Not surprising considering the entrance to the throat is 0.246+".

Things To Try Next Time and Down The Road:
-- try bullets that are actually sized to fit this sloppy throat, instead of "leftover" bullets.
-- try a slower powder that should be gentler on the bullet.

Question:
-- if the gas check was "catching" at the entrance to the 6BR's throat, what was different about the 6BR compared to the 22, 7, or 30 BR? They all had snug benchrest-quality throats.
-- one thing that is different about this 6mm Pacnor barrel is that it is a 3 groove. Most of my other barrels are 6 groove (except the 30 caliber Krueger which is 4 groove). Here's a theory -- what if the 3 groove does not support the nose of the bullet as well as a 6 groove, and allows the nose to tip in the throat? The tipping could make the gas check enter the throat cock-eyed?
-- yes, 3 groove barrels have been proven in CBA matches, but those matches typically employ long bullets at modest velocities. I'm shooting short bullets at high velocities.

[mtngun]

Today's goal was to sample different bullets and different powders in the Talldog. Unlike last week's "leftover" bullets, today's bullets were nose-sized to fit the TallDog's throat. The jam points were measured and bullets were seated to give 0.010" - 0.015" jam.

Here are the bullets we shot today.


I started out with WC845 (the 10.5" pattern on the lower left), then switched to Varget and continued with Varget for the rest of the day. I know from experience with the BR's that WC845 is very fussy while Varget is "decent," so I felt that Varget was a safe choice. I had planned to try more powders if any bullet showed potential, but no bullets showed potential.

As you can see all of the cast bullet loads were unacceptable. So then I did something that I rarely do -- I shot some jacketed bullets -- just to verify that there was nothing mechanically wrong with the scope or the rifle. The little Vmax bullets could not reach the rifling, and they rattled around in the .246" throat, yielding a 1.9" group. Not great, but still much better than the wild patterns produced by the cast bullets.


Observations and Conclusions
-- the 89 gr. bullet has a Miller Stability factor of 1.24 at my elevation -- only borderline stable -- but all of the bullet holes were round, so it did appear to stabilize just as the Miller formula predicted.
-- no errant gas checks hit the chronograph.
-- the Talldog's generous throat appears to eliminate the 6BR's errant gas check problem. That seems to confirm the hypothesis that gas checks were "catching" at the entrance to the 6BR's throat.
-- but the generous throat kills accuracy with cast bullets, even when the bullets are sized to be a good fit in the throat.
-- I'm going to assume that the throat is the problem with the Talldog. The rest of the chamber is smooth and snug. The Talldog's capacity of 33.5 gr. H20 is nearly identical to the proven PPC (32 gr.) and close to the 6x45 (30.5 gr.) so I think the cartridge itself is fine.
-- while the 3-groove Pacnor is smooth enough, and does not foul significantly, I'm skeptical that 3 grooves are optimal for cast bullets. This 3-groove barrel seems much fussier than my other barrels.

Where Do We Go From Here?
-- while there are always more load tweaks that could be tried, it's obvious that this chamber does not want to shoot cast bullets.
-- I may set the barrel back a thread or two, enough to replace the long, sloppy throat with a short, snug throat as per conventional benchrest practice. The barrel is useless in its current state so i have nothing to lose by setting it back.
-- but will a short, snug throat "catch" gas checks?
-- also, I happen to have a 14" twist, 6 groove Shilen barrel blank on hand. If time allows I may chamber it for 6mm Talldog with a short, snug throat. It'll be interesting to see if the 6-groove barrel "catches" gas checks. I bet it won't.
-- ditto for the 10" twist, 6-groove Shilen left over from the 6x45 project.

This Talldog project has disappointed me, but I suspect the fault is with the throat and perhaps with the 3-groove barrel, not with the cartridge itself.

[mtngun]

Stormy weather has shut down shooting for a while so this is a good time to stay inside the shop and work on gunsmithing projects.

I reground the old throating reamer for this barrel, which I believe was 1.5 degree per side x 0.245"+, to 1 degree per side x 0.2435". Or at any rate, that was what I was aiming for. Grinding shallow angles accurately is challenging and even final stoning can alter the angle slightly, so it actually came out closer to 0.7 degree per side x 0.243". No guarantee that such a throat will shoot well, but I'm willing to give it a try.

Bear in mind that the groove on this bargain basement barrel is actually 0.242", not the standard 0.243", so a 0.243" throat & cast bullet should be plenty big for the barrel. In theory.

[mtngun]

OK, so the Pacnor barrel was set back and rechambered, this time with a very tight throat. Also, I deliberately cut the chamber 0.020" shorter this time in an attempt to shrink the case volume closer to the PPC. The new specs:

-- 26" long barrel
-- 1.400" trim-to case length
-- 32.5 gr. H2O capacity (vs 33.5 for the original chamber, and vs. 32.0 gr. for the PPC)
-- 0.7 degree per side leade
-- freebore begins at 0.244" ( a .244 minus pin just barely enters)
-- 0.243" freebore approximately 0.070" long (based on how far a .243 minus pin can enter)
-- after cutting the throat, I used the 11 degree chambering reamer (discussed in the 6BR thread) to lightly chamfer the entrance to the throat. Emphasis on the word "lightly" -- I pushed the reamer in by hand until it made contact, then hand reamed approximately 0.020" past the contact point.

I still need to make a nose die with the 0.7 degree throating reamer, and the weather has been too stormy for accuracy testing, anyway. But I did a quick test to see if gas checks were coming off. 10 shots point shooting at 15 feet, using a 55 gr. GC pushed by 22 gr. IMR4198. There were no "extra" holes in the paper, so at the moment the 6BR's errant gas check problem appears to be cured. Unfortunately, I don't understand how it was cured?

10 shots, 10 holes.


I'm declaring a tentative victory over the errant gas check problem, and a victory at matching the PPC's case capacity. Now the question is, will the 3-groove Pacnor shoot well with the snug throat?

[mtngun]

Nose Die: I made a nose die to match the new "snug" Talldog chamber. First the die was reamed with a 0.237" chucking reamer, then the 0.7 degree throating reamer. Since this particular Pacnor barrel has a 0.236" bore, the 0.237" nose on the nose-sized bullets engraved the lands just right.

Bullets were sized in the 0.7 degree nose die, then in a 0.2435" push-through, and seated for 0.010" jam (except the Vmax was seated to 0.005" jam). 26.5 gr. Varget and Federal #200 primers were used with all of today's loads. This is the same powder/primer combo we used last time in the "sloppy" throat so it'll give us an apples-to-apples comparison.

For the record, I also changed the barrel bedding. The sloppy throat had been shot using two small, soft foam pads bearing on the barrel at the forearm, while the snug throat was shot fully free-floated. Previous testing suggested that the forearm bedding didn't make much difference but nonetheless I note it for the record.

The snug throat's target. No bragging groups, but better than the sloppy throat's shotgun patterns.

Note the velocity variation was not-so-good, and the heavier the cast bullet, the higher the velocity. This suggests the slippery cast bullets don't have enough resistance to make Varget burn well, and even the 58 gr. jacketed bullet had excessive velocity variation and vertical stringing.


On average, the snug throat cut groups roughly in half compared to the sloppy throat.


Conclusions and Observations:
-- the snug throat cut groups in half.
-- but 3 MOA groups are still unsatisfactory.
-- even the Vmax's 1.9" group is lousy by benchrest rifle standards.
-- Varget is not burning satisfactorily in this cartridge.
-- while no doubt accuracy could be improved by trying different powders, etc., this barrel does not act like it wants to be a tackdriver, and I'm out of ideas. I dunno if the current 0.244 x 0.7 degree throat is the ultimate throat, but I'm confident that it is a decent throat.
-- The Talldog cartridge has been proven in benchrest and is ballistically identical to the PPC, so I have confidence in the cartridge.
-- that leaves the barrel as the prime suspect. I don't keep exact records on how many rounds have been fired in my barrels, but I'm guessing this Pacnor has now fired 1500 - 2000 rounds. Even with the fresh chamber it may simply be past its prime?

Things To Try Next Time:
-- if I get bored I may experiment with different powders.
-- I want to try this cartridge in my 2 Shilen barrels.