Modifying bullets the hard way

[lloyd-ss]

I hope some of you find this interesting.
The majority of my shooting has been over a chrony during various development phases. Lots of different calibers and barrels and pressures.  Just trying to learn as a much as I could. During that testing I needed lots of different bullets, including ones that just weren't available.  The easiest way for me to try different bullets was to modify what I could buy and turn it into something that worked well.

A big disclaimer here!  This worked for me.  It's not an efficient way to make bullets, but it gave me what I wanted.  Most testing was over a chrony; limited accuracy testing.

This is what I did to get a medium weight .357 to use minimum air, get maximum velocity, and to get consistent velocities in my set-up.



This is a picture of the bullet, as purchased nominal 38 cal, 165 gn actual dia .3577 dia.
Top bullet is out of the box, bottom bullet was pushed thru the barrel to get dimensions.
Slugged thru barrel, nominal 38cal, 25” long, .3574 groove dia, .349 land dia, .039 land width, .144 groove width, 6 grooves, 1 in 16 twist, land height .0042.
Length of rifling contact .340 each land. (.080 sq in)
Length of barrel groove contact .300 for each groove. (.259 sq in)
Comment- The barrel has a nice skinny land compared to the wide groove.  That means less lead has to be displaced as the bullet is engraving (extruding)  itself into the barrel rifling.
But, the length of engagement of the bullet is way too much for an air rifle.  A lot of unnecessary friction and wasted energy (heat).



Top bullet is after all sizing operations, and the bottom bullet shows it after being pushed thru the barrel.
Bullets were modified as follows:
Center section machined down to .348 dia, gas check bands sized to .3575
Length of rifling contact .100 for each land. (.023 sq in)
Length of barrel groove contact .120 for each groove. (.104 sq in)
139 gn

The important stuff:
Original contact area .339 sq in
Contact area after modification  .127 sq in (62% reduction)

Original volume of lead extruded by the rifiling .00045 cu in
Extruded volume after modification   .000097 cu in  (78% reduction)

Less extrusion and friction mean higher velocities, so long as there is no blow-by and the rifling stays intact.
Remember, that was a PB barrel made to work at more than 5 times the pressure of airguns.



Here's the process I used:


bullet chucked in lathe



 
drilling out the base with 60 degree center drill




lead curls coming out




drilling complete




set up male drive center, live tail center, and bullet nose adapter




bullet in lathe and ready to turn




turning the center section of the bullet down




turning complete




sizing die and turned bullet




bullet pressed into die to depth.  Because die has taper, I pushed the bullet back out, not thru.




bullet ready to shoot.  It worked pretty darn well, but a thinner skirt might have worked better.

Again guys, this worked for me for chrony testing, and I can't really speak much to accuracy.
Lloyd

[rsterne]

How did the FPE of the 139 gr. modded bullet compare to the original 165 gr. bullet?.... I ask about the energy, as the velocity should have been higher just due to the reduced weight....

Bob

[JonnyReb]

 Just as brilliant as everthing else you've done for the Bigbore Lloyd, posts like these are eye opening not just because of the knowledge gained but because the concept itself causes one to realize their potential to create and discover. Bigbore is a very inexact science right now, what you've done in the above post probably encompasses the upper crust of techonlogy, testing in the way you have has probably narrowed down the proven components of efficient large bore AG's, although if industry decides to spend enough on R&D like they did for muzzleloaders we'll maybe see bullets with poly seals or other methods of making the projectile seal the barrel while minimizing friction and maintaining twist rates. Do you know if industry is aware of the benefit of narrow lands in barrels of this pupose? Jeff

[lloyd-ss]

How did the FPE of the 139 gr. modded bullet compare to the original 165 gr. bullet?.... I ask about the energy, as the velocity should have been higher just due to the reduced weight....

Bob

Bob,
Although the bullets didn't look that too far off dimensionally in their original condition, they were fairly hard bullets and were difficult to push thru the barrel.  The mod was good for an additional 35+ fpe at 1400 psi while maintaining the same air consumption. A softer bullet may have shown less improvement.

Where I really saw the improvement was in the consistency of the velocity when shooting at low pressures. At higher pressures it didn't make as much difference in vel consistency.

But I think a proper fitting soft lead bullet would do the same thing.  The important point to me is that bullet fit is very complex, a lot of things will work, and that the wrong bullet can lead to disappointment.

Lloyd

[lloyd-ss]

........... Bigbore is a very inexact science right now, what you've done in the above post probably encompasses the upper crust of techonlogy, testing in the way you have has probably narrowed down the proven components of efficient large bore AG's, although if industry decides to spend enough on R&D like they did for muzzleloaders we'll maybe see bullets with poly seals or other methods of making the projectile seal the barrel while minimizing friction and maintaining twist rates. Do you know if industry is aware of the benefit of narrow lands in barrels of this pupose? Jeff

Jeff,
Thanks for the kind words. 
I think all the well known/respected  BB makers have found combination's that work well for them and are proven winners. I would be a fool to argue with their successes, experience, and knowledge. They know what they are doing.

But I do agree that it is an inexact science with many possibilities.  When I was looking around for barrels I heard many different opinions about rifling and fit and as we've all said, you have to learn your own gun and find out what it likes.  My personal feeling on rifling is that it should have to displace (engrave, extrude) the least amount of lead to seal the pressure, and to not shear the rifling.  Have you ever hammered a bad fitting bullet out of a barrel? A .357 bullet has a cross section of .10 square inch, so at 2,000 psi, it only has 200 pounds of force trying to push it out of the barrel and accelerate it to max vel before it leaves the muzzle. A fair percentage of that can go to just pushing the bullet thru the barrel and not adding to the velocity.


Back to the rifling, I got a .308 "air" and a .357 "air" barrel from a very respected barrel manufacturer.  I slugged both barrels and the configuration of the rifling land was the same for both calibers.  That meant that the groove in the .357 was 20% "roomier" than in the 308, which made it easier for the lead to extrude in the 357.  Compound this with the fact that area of a 308 is only .075 sq inches.  That 200 pounds of force working on the 357 bullet is only 150 pounds on the .308, plus the 308 had more constricted rifling.  That .308 barrel did not perform well.

I have not had the pleasure of using many of the more appropriate special purpose air rifle bullets, but I definitely need to do more of that.
Lloyd 

[Brushy Bill]

A lot of the SASS "cowboy shooters" use a
softer lead bullet. I have often thought about
ordering a barrel liner/blank from Track of the
wolf. Making a brass lap that would just lap
the lands of the rifling and not the grooves
to cut down on the force of the lands cutting into the
bullet.
I think that reducing the land height to 0.0015 or so
would still provide plenty of grip on the bullet.
Then using a bullet like this one;

.38 (0.358 diameter) 105 grain.
This bullet has very little contact area considering the
width of the lube groove.

From what I have seen from testing that Robert Vogel has
done using flat based bullets that he cast, I don't really see
a need to have a hollow base.

Does any of this sound reasonable to you?

[lloyd-ss]

Brushy Bill,
I like the idea of narrower and/or shorter lands.  I wonder if anyone has tried a microgroove barrel (Marlin type)  on an air rifle?
I think Track of the Wolf barrels come from Mike Sayers in KY, and those are hammer forged out of chrome moly steel and are pretty tough.  Trying to lap out a few thou might be a real chore.
Lloyd

[Brushy Bill]

It would be for sure a time consuming process.
With the diamond lapping compounds available
it could be sped up some, but it would take a while.

I have had some pretty good luck making adjustable diameter
brass laps. Once you get one loaded up, they can remove
some steel. Especially since you are not changing them out and
starting the loading process over as the diameter grows.

They could be used in a slow cordless drill since were not
dealing with the grooves. Then, the final lapping/polishing done
by pushing and pulling.

It is neat to know those barrels come out of Kentucky, a lot
of good things have came from there. My parents for example.
And some good Bourbons.

Thanks for responding, think I will try a carbine/long pistol
length barrel first.

It sure would be nice to be able to buy a box of 500 bullets off a shelf
and maybe resize them. (if needed)
Compared to the hand poured big bore bullets available.
Not to take anything away from them, they seem to be very good
quality.

[Jaymo]

Interesting post, Lloyd. Especially since you did lathe work. I like any excuse to fire up my lathes, mills, and shaper.
I like the idea of the Marlin Micro-Groov barrel. If I can get one in .44 or .45 caliber, I could rebarrel my 909 with it and use .429 or .452 bullets, instead of .454. There's a lot better selection of the former 2 than the latter 1.
I'd love to try my Lyman Devastator .452 hollowpoint through a Marlin .45 Colt barrel in my 909.
Or, I may get another Lyman Devastator 250 .429 HP mold and use my shaper or mill and cut down the tops of the mold blocks to get the bullet weight down to somewhere between 150-200 grains. Fill the bullet nose with soft bullet lube, to initiate expansion, and have fun.

[lloyd-ss]

Interesting post, Lloyd. Especially since you did lathe work. I like any excuse to fire up my lathes, mills, and shaper.
I like the idea of the Marlin Micro-Groov barrel. If I can get one in .44 or .45 caliber, I could rebarrel my 909 with it and use .429 or .452 bullets, instead of .454. There's a lot better selection of the former 2 than the latter 1.
I'd love to try my Lyman Devastator .452 hollowpoint through a Marlin .45 Colt barrel in my 909.
Or, I may get another Lyman Devastator 250 .429 HP mold and use my shaper or mill and cut down the tops of the mold blocks to get the bullet weight down to somewhere between 150-200 grains. Fill the bullet nose with soft bullet lube, to initiate expansion, and have fun.

Jaymo, be careful what you ask for!
First, about "reasons" to run the machinery.... my wife bought a new toilet paper holder that the big double rolls of paper wouldn't fit into.  I went to the garage and pronto made a neat little spacer and now the rolls fit perfectly.  I thought that would have justified the expense of the lathe, but she still doesn't quite agree.

You started the wheels turning about the big cal micro groove barrels.  I looked at Numrich gun parts (one of my favorite sites for deals!!!) and darn if they don't have surplus  45-70 Marlin 1895 micro groove barrels for $64.50.
http://www.gunpartscorp.com/catalog/Detail.aspx?pid=939340&catid=3977
 They are only 22" long, and after you cut the chambering off, maybe have 20"?  A sweet but loud carbine barrel.  Somebody on here just got a Jack Haley with a 16" barrel, I think?   If Jack can make it work, then it otta' work.

They also have 30-30 chambered microgroove barrels 20-1/2" long, so maybe 18-1/2" after mods?

This is a serious dilemma.  

So are you buying first, or am I? and which one?
I really want to try the microgroove rifling with air.  I think it will be a winner!

Lloyd