Shooters Forum banner

1 - 9 of 9 Posts

·
Registered
Joined
·
166 Posts
Discussion Starter #1
I've been working up loads for a barrel I received as part of a trade, 1-8 twist 6mm AI. It shot very well with 87 grain bullets and almost as well with 70 grain bullets, but 20% of the 70 grain rounds tumbled. Switching to slower powders the tumble rate decreased to 10%. There was no in between, accurate or tumbling.

With a 1-8 twist, the lighter bullets should have been easily stable.

Examining the barrel with a 10X loupe at the breach and comparing it side by side with a new barrel, the lands appeared rounded at the throat with a textured appearance indicating a significantly worn throat.

I found an old Springfield Armory test in the Small Arms Review Reference Library, available online, that used an erosion test criteria of 15% yaw or a velocity reduction of 200 fps. In no case was the velocity reduction reached before the yaw. Yaw would be indicative of a loss of stability.

Perhaps the high acceleration of the lighter bullet using faster powders causes them to skip the worn rifling and not spin up normally.

So I try even slower powders, just shoot heaver bullets, scrap the barrel or set back the breach and re-ream.

(M16A1- Erosion Test on 5.56mm Rifle Barrels- Small Arms Weapons Systems Study (SAWS). Technical Report, Springfield Armory. 30 June 1967, English Language.)
 

·
Elk Whisperer (Super Moderator)
Joined
·
11,187 Posts
Maybe those bullets aren't tumbling so much as coming apart? That 70 grain is spinning at a pretty good clip.

My 1:9 .243 shoots up to 105's "hunting class" bullets quite well but heavier "target" bullets are too long for that twist. On the other hand it shoots 58 grain bullets better than I can hold it.

RJ
 

·
Registered
Joined
·
289 Posts
I designed a 6mm wildcat I call my "244SP-EXP." Think 6mm Rem AI on steroids, or a 6mm Gibbs would be a more accurate description. It has a 1:8" twist, 26" bbl, and shoots everything from a 55gr. Ballistic tips and 58gr. Vmax, to 100gr. Partitions and 100gr. Interlocks usually into groups .5" or less. The 70gr. Speer TNT is incredibly accurate in my rifle and is the varmint bullet I shoot the most. It's mostly been a safe queen it's entire life, so I cannot attest to accuracy with a hot barrel.
 

·
Registered
Joined
·
166 Posts
Discussion Starter #6
Decided to Scrap the Barrel

The bullets were loaded at the low end, probably around 3400fps, well within typical velocities for varmint bullets and an 1-8 twist. They are definitely not coming apart, as you can see sideways entries into the target. I've used three manufactures of 70 grain bullets with exactly the same results. This was confirmed over several range sessions during trouble shooting.

Obviously this is counter intuitive to what we usually observe, taking into consideration of the original .244 Remington twist issue of too slow a twist for heavier bullets causing stability issues which resulted in the rebranding of the cartridge as 6mm and manufacturing faster twist guns for it. They went from 1-12 to a 1-9 twist expecting people to shoot 55 grain to 100 grain bullets. My 6mm AI is simply a 6mm Remington with a blown out shoulder loaded to 6mm Remington specs.

The heavier bullets which did not tumble are accelerating slower and have a longer bearing surface which may have something to do with getting them spun up properly. Just a guess, but until I got a good look at the throat comparing it side by side with a new barrel and, I couldn't make any sense of it. The first half-inch to an inch of the lands are clearly worn down, lacking the sharp definition of the new barrel. The throat has a textured appearance indicating the surface is no longer smooth and polished but damaged.

I normally work with new barrels so its all new to me. Since there is a major problem with the barrel and it has problems shooting, I have to conclude the problem is due to the barrel. It explains why it was thrown into the deal, i.e., it is worthless except if you are a gunsmith with time on you hands to do major surgery.

I intend to scrap the barrel and get a new one and chalk it up to experience.
 

·
The Shadow
Joined
·
7,932 Posts
1) The bullets were loaded at the low end, probably around 3400fps, well within typical velocities for varmint bullets and an 1-8 twist. They are definitely not coming apart, as you can see sideways entries into the target. I've used three manufactures of 70 grain bullets with exactly the same results. This was confirmed over several range sessions during trouble shooting.

2) Obviously this is counter intuitive to what we usually observe, taking into consideration of the original .244 Remington twist issue of too slow a twist for heavier bullets causing stability issues which resulted in the rebranding of the cartridge as 6mm and manufacturing faster twist guns for it. They went from 1-12 to a 1-9 twist expecting people to shoot 55 grain to 100 grain bullets. My 6mm AI is simply a 6mm Remington with a blown out shoulder loaded to 6mm Remington specs.

3) The heavier bullets which did not tumble are accelerating slower and have a longer bearing surface which may have something to do with getting them spun up properly. Just a guess, but until I got a good look at the throat comparing it side by side with a new barrel and, I couldn't make any sense of it. The first half-inch to an inch of the lands are clearly worn down, lacking the sharp definition of the new barrel. The throat has a textured appearance indicating the surface is no longer smooth and polished but damaged.
.
A few things to think about, for what they may be worth to you.

1) "Probably" means you don't know, so anything is a guess. That said 3400 @ 8 twist is over 300,000rpm. That is more than most bullets can sustain, especially with a rough barrel. So it isn't really shocking they would have trouble. 70gr bullets are nowhere near the same length, nor construction. So what does "coming apart" actually mean? We know the obvious, but if they simply had a not unusual Core/jacket slip, then balance goes out the window, and they will over-turn and do all manor of strange things.

2) The twist isn't for "heavy" bullets, that just gets said for ease of generalization; Twist is for length stabilization.

3) Heavier bullets won't be going as fast ultimately, and will be spinning slower; which can help with stability if on the edge. They won't be accelerating more slowly, at least with any significance. The bearing surface depends on specific construction of course, but it's a double edged sword. On the one hand, it can offer more area to have rifling engage. On the other hand, it affords more area of the jacket to damage from a rough bore; and can lead to more likely hood of causing bullet destruction.

Let us know what you do with the new tube(what chamber, etc) and how you like it.

Cheers
 

·
Super Moderator
Joined
·
9,551 Posts
Never really are sure of the causes....just not shooting in a dedicated lab (and even then it'spretty much theory).

Even this isn't exactly all about tumbling (and is a long ramble),it might simulate some thoughts.

1. Do know you can over drive a fragile bullet to the point where it comes apart in mid air(kind of neat looking...like a split second of "gray rope"in the air), but that results in a no-hole shot.I use "fragile"as I've had it happen with faulty bullets that should have been strong enough and with very fragile bullets driven too fast.

Some bullets come with a velocity range warning on the box...likely a good idea to follow it. It may not be there becasue of the resultson game,but becasue you can actually "grenade" them on the way there.

Faulty bullets can also "grenade".

(Long tale, and I accidently made the bullets faulty,but illustrative):

Swaging .357" .158gr. bullets down to .352" to feed a .351WSL. Looked great,well swaged, balanced. Commo to have fliers, and occasionally key-holes, and once in awhile one would just "grenade" mid-flight.
Turned out that lead does not spring back like bullet jeckets do and the jackets were slightly loose to the core (not "rattle loose"so you could hear it/detect it). Swaging them down to .349"and then bumping them UP to .352" cured the problem.

Same bullets,same final diameter, same rifle,same speeds.

What I don't know (well...just a small slice of all the stuff I don't know):
A. Were the fragile bullets coming apart only becasue of bullet jacket thickness/strength?
B. Was the rifling creating stress lines/scoring that provided fault-lines for failure?
C. Why didn't obturation "fix" the loose jacket/core problem?

2. Do know that a well eroded barrel throat tends to be less accurate and that erosion slowly progresses down the bore becoming more and more inaccurate.

Only once have I shot a single rifle enough to actually wear it out past any useful accuracy and gotten occasionaly key-holes from loads/bullets that started out very accurate.

Normal proceedure was to change loads/bullets as accuracy from the erosion increased and accuracy decreased. Going to longer bearing area bullets that would still stablizeand seating them far enough out kept them going.

With a 1:12 twist .224 barrel, the longest bearing area bullet back then was the Speer 70gr. semi-pointed flat base. It'sreally not longer than spitzer botat tail 55gr....just blunter and with more bearing area,which allowed it to be seated way out...with at least a part of the bearing area past the eroded section. Would shoot well, at least until he eroded area progressed.

(YEah....it needed a new barrel well befor that point...but money was tight,was already a custom barrel when I started (on an Arisaka 38 action) which would be expensive to have made.)

A.How much did the reduced velocity required for the heavier bullet help?
B.How much did the thicker jackets of the heavy weight (for the time) bullet help?
C. How much did having at least part of the bearing area in a "clean"section of barrel help?

3.(Using the above rifle). HAd some ammo,loaded to the orginal "clean barrel" length, that was used in the early non-eroded shooting and had occasiona access to a chronograph.

(Which was a real pain to use...2 wire/paper screens that needed to be replaced after every shot....needed a big dry-cell battery (or car battery) to run...no direct read out..etc)

Know that as erosion progressed,velocity of the orginal ammo decreased.

A.was it becasue the erosion allowed more gas blow by?
B. Was it becasue the lack of inital resistence to engravement changed the buring of the powder charge?
C.Was it becasue the rough eroded surface increased resistance/bullet friction?
 

·
Registered
Joined
·
1,298 Posts
Short bullets (little bearing surface) and fast twists always seemed to be the issue at the range back in the day. I completely wore out a 6 X 63 throat and never had a bullet tumble (1 X 9 twist). Accuracy went down the tubes no matter what I tried and the chamber eventually got goobered up which made it all moot. I always had issues with light bullets in that rifle though, even before it went to heck. I think it was RPM's with such an overbore cartridge.

Just a thought.

Joel
 
1 - 9 of 9 Posts
Top