A Tale of BPCR BoattailsDan Theodore


 

Since the mid-18th century, maybe earlier, marksmen have experimented with boattail bullets (BTB).  The earliest record this crank is aware of is from the 1840’s.  Muzzle loaders were used to launch them back then with black powder.  The allure of using boattails is reduced bullet drag so the bullet shoots flatter and is less affected by the wind as it spins down range.  For BPCR Silhouette an added attraction is the possibility of shooting a light for caliber bullet that will not be unduly affected by the wind.  This opportunity is especially attractive for the 45-caliber rifles.  Most successful riflemen are shooting bullets in the 530 to 560 grain range in the silhouette game when using 45-caliber rifles.  The initial concept was to design a BTB that would weigh between 450 and 475 grains and have a ballistic coefficient as high as a 560-grain bullet.  The reduction in recoil will be substantial and for a 45-70 the lighter BTB can be launched at a much higher muzzle velocity (MV).  With 70 grains of Swiss 1.5 the estimated MV for a 450-grain BTB is in the 1,290 fps range.  It has been observed by many a rifleman that at the longer ranges higher MV is necessary to reduce the nasty affects of headwinds and tailwinds as well as the vertical dispersion at the ram line caused by velocity variation.  By shooting a BTB one can accomplish the reduction in head/tail wind affects and at the same time have a more comfortable rifle to shoot.  Certainly much work remains, but today’s first go using a 45-90 Pedersoli John Bodine with the new BTB from Old West Bullet Moulds has this crank excited about the potential.

 

First, the BTB was designed for use in a 45-70 but this crank was a little behind with the program and couldn’t wait for that rifle to be put together.  The bullet was designed a few months ago.  The bullet diagram sent to Bernie at Old West Bullet Moulds (allisonmonument@aol.com.)  Bernie cuts cherries to make his moulds so any of you that are interested in giving the BTB a try Bernie can cut the same bullet for you.  He did an excellent job of executing the design.  I couldn’t ask for more.  When I first cast a few bullets last week the joy of seeing exactly what had been designed drop from the mold was a real treat.  Bernie’s molds are beefy, made of brass and very well constructed.

 

The bullet specs are best described by a diagram instead of a 1,000 words.  Here it is:

 

 

One important parameter to keep in mind when experimenting with BTB’s is that a hard antimonial alloy must be used.  The bullets tested today were cast out of Lyman # 2 alloy, 90/5/5 Pb/Sn/Sb.  A 50/50 mix of Pb/Linotype should also be most satisfactory.  If a soft alloy is used like 20-1 the base will distort with diminished accuracy as a result.  This crank knows of a few shooters that used 30-1 alloy and reported excellent accuracy with their BTB’s.  I’m sure they had excellent accuracy only the bullet that exited the muzzle no longer had a boattail.  It had a flat base from the black powder whack to said base.  When using antimonial alloys the bullets must be aged for a minimum of 48 hours to achieve a stabile hardness.

 

The loads tested today all had neck tension in the 0.003” range.  All bullets were seated so that the base and next driving band were in the case.  No wad was used between powder and bullet.  The trickiest part of working with these bullets is the lubing process.  If a lubesizer is used a lot of lube is wasted as the lube fills in at the BT as the bullet is pushed through the die.  Dip-lubing is the way to go, but there can be no lube on the BT or accuracy will suffer.  A number of lubing techniques were tried over the course of the past week with a final technique that works well and is quite quick.  For those of you familiar with the grooveless bullet dipping process I’ve developed you will better understand what was done.  The dipping tub was filled with White Lightening BPCR Grooveless Bullet Lube so that when the base of the bullet touched the tub bottom there was a 0.050” ring of lube forward of the first grease groove.  The bullet was lifted out of the tube, base wiped on the edge of the tub to remove some of the excess lube and all of the lube finger-wiped off the BT and then finally the base wiped on a paper towel.  As you can see from the below picture the technique works quite well.  The lube used was specially formulated to work in this type of application.  Standard lubes might not work very well with this BT and they certainly do not work at all with the grooveless, bore-diameter bullets that a number of cranks are using.  Yet again the saving of a 1,000 words with a couple of pictures.

 

   

 

When the bullets were seated with neck tension, the excess lube was wiped from the bore-riding section.  For no neck tension testing the lube forward of the first grease groove will be left on the bore-riding section  The seating process when using a die scrapes off a good bit of the lube on the bore-riding section.  That is why it was wiped off.

 

The load components used for today’s testing were:

 

Buffalo Arms 45-90 stretched brass – most excellent brass

Federal 150M Large Pistol Match primers

No over-primer wads or over-powder wads were used

Swiss 1.5 powder

White Lightening BPCR Grooveless Bullet Dipping Lube

Boattail Bullet: 477 grains cast with Lyman # 2 alloy

 

All of the charges used were weighed on a digital scale and poured into the cases.  The charge range was from 55.0 to 72.5 grains in 2.5 grain increments.  The 55 grain load filled the case about half full.  The 72.5 grain load was about 98% load density.  None of the test loads had powder compression.

 

The barrel was wiped between shots with a 2” diameter patch damp with distilled water.  The patch was pushed slowly and smoothly down the barrel with a nylon brush attached to a custom made fiberglass cleaning rod.  After the patch fell from the muzzle the brush was drawn back to the breach and then pushed to the muzzle and back again.  The next round was loaded without any other procedure.

 

As expected the lower range of powder charges produced wide velocity variation over the Oehler chronograph but the reason for testing the low density loads was to investigate the shape of the bullet holes through the heavy corrugated cardboard at 100 yards. The higher density loads also produced high MV variation.  It was believed that the MV variation would trend lower as the charge increased, but that did not happen.  Iron sights were used with SR1 repair center targets at 100 yards.  The lower range charges produced slightly elliptical holes through the targets that gradually diminished as the MV increase from the larger charges.  The 65-grain charge produced an average muzzle velocity of 1,109 fps.  At that MV the bullet holes through the cardboard were round.  Accuracy was good with some 5-shot groups going below 1 MOA.  Below are some of the groups and all of the chronograph data.

 

Powder Charge

Average MV fps

Extreme Spread

Standard Deviation

55.0

971

17

6

57.5

999

21

8

60.0

1,028

14

6

62.5

1,066

14

5

65.0

1,109

14

5

67.5

1,142

28

11

70.0

1,173

16

6

72.5

1,196

29

11

 

The 65.0 grain load shot well and had decent velocity statistics.  Below is a scan of the target.  The second group formed after a 10 minute cease-fire.  This pattern is typical of a string of fire interrupted by a cease fire.  The rifle was wiped after the 3rd shot, group on the left and after the commence-fire was given the right group formed with the last two rounds.

 

 

The best BT group of the day was punched by the 70.0 grain load.  As you will see it needs to be shot in a silhouette match.

 

 

I believe that the 45-70 Browning will do a better job of reducing the velocity variation as the 45-90 has too much case capacity for this type of bullet and loading technique.  Even though some of the 5-shot groups were excellent the velocity variation was too high to qualify as match-grade loads.  When serious testing commences it will involve loads with neck tension as well as finger-seated loads with the bullets hard into the lands.

 

In closing, it should be said that this loading technique is an advanced technique.  There are a number of variables that must be controlled or accuracy will suffer severely and as such it is not recommend to any but the hardcore rifle crank who has a good deal of patience and determination.

 

Cheers,

 

Dan Theodore

2-24-05