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BountyHunter
Reply with quote  #1 
Question for the dasher shooters.

Ran across a couple very good 1k shooters using Dashers that say they are getting serious throat erosion at 400 rds.

To the point of rechambering at 400-500 rds.

They are winning more than their fair share and one is a smith.

Has anyone else run into this issue with the Dashers?

BH
Moderator
Reply with quote  #2 
Well, this raises a whole can of worms. Some of the sub-issues that may be at play are: bbl manufacturing, seating depth, powder "heat of explosion", caliber choice, velocity, moly-induced erosion, and cleaning technique.

Richard Schatz probably has as much experience with the 6mm Dasher as anybody (having one of the first Dashers ever built). He set a 6-target 1K world record with something over 1500 rounds on his barrel. He shoots naked bullets, RL 15, and jumps his pills. He runs 32.5 RL 15 at 3025 fps.

James Phillips notched a couple world records with a 22 Dasher. He was disappointed with his barrel life, and felt the accuracy was going at around 900 rounds--well before he expected. He shot a very hot load (36.0gr H4350 at 3400 fps) with moly bullets and jammed .010" into the lands.

Duane Capehart shoots a load of about 33.0 H4895 at about 2950 fps, not a lot faster than some guys are driving the standard case. He shoots naked bullets, jumped about .005-.012. Barrel life has been about what you'd expect--around 1500 rounds.

Now, mind you, this is all anecdotal, but when I hear about accelerated barrel erosion in Dashers, it seems there is a correlation with: 1) very large volume of powder; 2) "Jam" seating; 3) Use of Moly (yep); 4) Barrels from particular manufacturers (land height and steel quality seems to be an issue); Caliber--the 22 Dashers wear out faster than the sixes. Conversely the guys who get long barrel life usually run a lighter load, at slower velocities, and there may be a beneficial correlation with jumping the bullets.

When you think about it, all these things--less powder, lower velocities, and seating away from the lands, translate into lower peak pressures and less barrel heat.

Moly is the puzzler for me. It is supposed to extend barrel life. I suspect what may be going on is that the moly shooters are cleaning less and getting more carbon build-up which roughs up the throat area and creates a heat sink. This is just a theory of course.

Also, when using moly, most people need more powder to reach a given velocity. That may explain a great deal. More powder = more heat.

There is also the possibility of actual moly "poisoning" of the barrel leading to acidic corrosion. (Moly + water vapor can create acid.) I had this happen to me once with a pistol barrel to which I applied a liquid moly coating from Sentry Solutions on the bore and chamber. After about 800 rounds the chamber and throat were badly pitted and scarred. I sent the barrel back to HK (known for very good steel) and they said they'd never seen anything like that before. Keep in mind that pistol chamber pressures are way below the 60K+ psi you'll get in a match rifle.

I hesitate to speculate about moly because Norma and others have done tests that suggest moly should reduce throat wear dramatically. But Norma's tests involved bullets coated with wax and moly and there pressures were lower than what most Dasher shooters run. I suspect Norma also used sensible and uniform cleaning practices and there was never an issue of moisture contamination.

Henry Childs has a theory that powders such as Varget, which have a very high heat of explosion, wear out barrels faster than powders, such as N160, which have a much lower heat of explosion. There was a long thread running about that issue some months ago.

Bottom line, I would like to know the make of the barrels that are wearing out after 500 rounds, I'd like to check the powder charges, and I'm very curious about the cleaning methods used. I've seen a couple 6.5-284 barrels worn out prematurely by over aggressive cleaning.

EROSION on a Standard 6BR:
FWIW, my std 6BR PacNor has shown less than .003" throat "erosion" in 450 rounds, as measured by ogive contact at the lands with a Stoney Point tool. There's a fudge factor in that number because you really should measure with the same bullet every time--but I usually measure five bullets and take an average. I shoot naked bullets, use Wipe-Out for all major cleaning, and I've never put a brush through the bore. I generally shoot 40-50 rounds between cleaning, though I might run a couple wet patches through after 25 rounds if I'm changing loads. I've never used JB or Iosso. I shoot 30.3-30.5gr Varget with 105 Scenars jammed .010".
jb1000br
Reply with quote  #3 
Well, if my guess is right...

barrel is kreiger
powder = varget 33.5 IIRC
107smk at about 3050
cleaning with brushes and MX 5BMG
NO moly

JB
BountyHunter
Reply with quote  #4 
Jason

could be right, but they had several Dashers.

Kind of surprised me with the talk of throat erosion.

BH
Rust
Reply with quote  #5 
It's a vicious circle, add more powder there is more thermal energy released which means there is more and hotter gas. More and hotter gas in a given volume means more pressure. More pressure means more efficient transfer of heat.

Land and groove profile add or subtract from the mix, lands of less width for a given height would erode faster, simply a matter of the amount of surface area to volume within a land. Probably why five and three land barrels seem to last longer. Wider lands, better volume to surface area ratio. And yes, anything that sticks up in the path of a mass of very hot gas traveling at extreme speeds is for all intents and purposes a fin in a heat transfer system. Thin fins for height transfer heat more readily.

Worse case scenario would be a hot powder high pressure load in a barrel with narrow lands.
mikecr
Reply with quote  #6 
I think(but don't know) that moly by itself cools the flame some through latent heat of vaporization. But, this causing velocity to drop, people add more powder, which has a greater effect on barrel life than the little bit of cooling offered by moly.

Theory 2: Bullet timing changes expansion ratio, slowing velocity, causing people to add more powder.

Either way, more powder and overall heat is added.
6Dasher
Reply with quote  #7 
I shoot a Broughton 1:8" using N140 for naked 88LD Bergers and 105gr Scenars. I noticed a rough area from the throat to about 2-3" into the barrel; JB will only keep it smooth for about 100 rounds. I've shot 1400-1450 through it so far. The peak accuracy it had the first 600-700 rounds is gone, but it still shoots nice little groups for f-class and such.

BTW: I shoot the 88LD at 3300fps and the 105gr at 3050fps. Moderate pressures with cratered primers (no sticky cases).
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