An Analysis of and Recommendations for the New NRA/Whittington Center Pit Safety Rules for BPCR Longrange Matches


The new rules that govern minimum bullet weight and muzzle velocity by caliber do not address the critical issue of pit crew safety appropriately.  As a matter of fact, the 45-caliber and 50-caliber minimum requirements should not be allowed.  At the same time the 38-caliber minimums are overly restrictive and do not rationally compare to the minimums for the larger caliber rifles.  The critical parameter that the rules attempt to control and set minimums for are the terminal trajectory angles at the pits of the various calibers listed in the rules.  It is evident from a review of the chart below that basic exterior ballistic knowledge has not been applied in these determinations, nor have any live-fire tests been conducted and results applied to assure pit crew safety.  The new bullet weight and muzzle velocity restrictions by caliber are:

 

NRA/Whittington Center Pit Safety LR Load Minimum Requirements

 

Caliber

Min Bullet Wt (grains)

Min Bullet MV (fps)

38

400

1,300

40

400

1,280

44

450

1,240

45

510

1,200

50

600

1,200

 

As was said earlier, the minimum 45 and 50 caliber loads should not be allowed in Longrange Matches as the MV’s are too low and will produce unacceptable trajectory angles at the pits when shot from the 1,000-yard line.

 

First a discussion of some basic exterior ballistics theory and how it can be used to insure pit crew safety in a way that treats each of the calibers listed in the new rule in an equitable, rational manner.

 

The equation for Ballistic Coefficient (BC) is the simple ratio of bullet sectional density divided by its drag coefficient.  The equation is:

 

BC = Bullet’s Sectional Density / Bullet’s Drag Coefficient

 

We can think of Bullet Sectional Density (SD) as the “force”, when the bullet is flying down range, which reduces the slowing effects of aerodynamic drag.  A high SD means the bullet retains its velocity better; therefore the bullet shoots flatter with less wind deflection.  A Bullet’s Drag Coefficient can be though of as “air brakes” that slows the bullet.  The larger the SD “force” the higher the BC and the smaller the drag the higher the BC.

 

First let’s investigate the bullet’s Sectional Density.  Sectional Density is defined as bullet weight divided by the caliber squared.  Bullet weight is simply the weight in grains divided by the number of grains per pound.  The equation looks like this:

 

Bullet Weight (lbs) = Bullet Weight in grains / 7000 grains per lb

 

Now the SD can be written as:

 

SD = Bullet Weight (lbs) / Caliber Squared

 

An example for use in this discussion is the 600-grain, 50-caliber bullet in the new NRA/Whittington Center LR Load Rule.  Its SD is:

 

Sectional Density (50-cal bullet) = (600 gr / 7,000 gr/lb) / (0.512” x 0.512”)

 

Sectional Density (50-cal bullet) = 0.327 lbs per square inch

 

With the above result in hand we can say that any bullet, as long as the Sectional Density is the same as the 50-caliber’s, no matter the caliber from the above NRA Rule Chart, and if the bullet designs are all the same the BC of all of the bullets will be similar.  That is, however, not quite accurate.  It is, however, sufficient for this discussion.  Actually, the 38-caliber bullet will have the highest BC of all calibers in the NRA Rule that have a 0.327 lbs/in^2 sectional density because it has the lowest drag coefficient.  Since BC is the ratio of SD divided by Drag Coefficient (DC) and the bullets in this example have the same SD for all calibers and the 38-caliber bullet has the smallest DC, it MUST therefore have the highest BC.  It should be noted and understood that BC defines terminal trajectory angle at the pits and wind deflection without regard to caliber.  Another way to say this is that if a 38-caliber bullet has a higher BC than a 50-caliber bullet and they are launched at the same MV, the 38-caliber bullet will have a flatter trajectory at all distances.

 

Bullet weight has incorrectly been thought by some to be all that matters concerning trajectory and wind deflection for a given muzzle velocity.  Bullet weight alone does not predict these two parameters: trajectory angle and wind deflection.  This misperception has permeated BPCR competition with unfortunate affect.  It is time to put an end to these misperceptions and not allow them to have a negative affect on BPCR competition.  The science of exterior ballistics, that clearly shows what is safe for the pit crew, should be the only deciding factor not ignorance, prejudice or personal agenda.

 

If all of the calibers listed in the NRA pit safety rule had the same Sectional Density then the following chart contains their weights given that they all have the same SD as the 50-caliber’s minimum bullet weight.  The bullets would all be about 1.30” long.

 

Equivalent Bullet Weights for Same Sectional Density

 

Caliber

Sectional Density (lbs / in^2

Bullet Weight (grains)

38

0.327

322

40

0.327

381

44

0.327

455

45

0.327

480

50

0.327

600

 

In the above chart the 38-caliber bullet would actually have the highest BC due to its lower Drag Coefficient with the 50-caliber bullet coming in last in the BC ranking because it has the highest Drag Coefficient.  That means if all of the above defined bullets were shot at 1,200 fps the 38-caliber bullet would shoot the flattest and be least likely to cause pit safety concerns.

 

To show how ineffective the new NRA/Whittington Center rule is in assuring pit crew safety, logically and rationally the 50-caliber minimum bullet weight and 1,200 fps MV translate into allowing a 322-grain, 38-caliber bullet launched at 1,200 fps to be allowed for LR matches.  Certainly both loads should be banded from competition as they produce too steep of a trajectory angle at the pits when fired from the 1,000-yard line.  The estimated trajectory angle for both loads at 1,000 yards is 4.2 degrees below horizontal.  Neither of the loads should be allowed on the firing line for LR matches.  One could shoot a 38-55 launching a 322-grain bullet at 1,200 fps and the pit crew would be just as safe as if a load with the new 50-calber minimum was used.  Certainly it is easy to see how ridiculous the 38-55 load is for shooting at 1,000 yards, but it does not seem to be clear to some that the 50-caliber load is equally ridiculous and unsafe.  Actually the 50-caliber load could arguably be called more dangerous as a 600-grain bullet bouncing around in the pits is much worse than a 322-grain bullet doing the same.  The 50-caliber bullet would have a 1,000-yard terminal energy of about 689 ft-lbs while the 322-grain 38-caliber bullet would only have about 370 ft-lbs of terminal energy.

 

A more rational, safer rule for pit crew safety in BPCR LR Matches would be to restrict all calibers to a minimum muzzle velocity of 1,250 fps with the following bullet weight minimums based on a 500-grain, 45-caliber Government bullet which has a sectional density of 0.341.  Barrel twist restrictions are also necessary to further ensure pit safety.  Stabile bullets require proper barrel twist-rates.  For LR the barrel twist-rate requirements are higher than for BPCR Silhouette.  If a bullet is not properly stabilized its BC plummets which causes a steep trajectory angle at the pits.  They are included in the table below for the scientifically based set of rules that help assure pit crew safety.

 

Suggested LR Minimum Bullet Weights by Caliber - 1,250 fps Minimum MV

 

Caliber

Sectional Density (lbs / in^2

Bullet Weight (grains)

Twist

38

0.341

335

13

40

0.341

397

16

44

0.341

474

18

45

0.341

500

20

50

0.341

625

26

All bullets from the above chart would have a 1,000-yard trajectory angle of 3.7 degrees below the horizontal when shot with a MV of 1,250 fps. This scientifically developed and proposed restriction of bullet weight and MV by caliber will help ensure pit crew safety far better than the current rule that unfairly penalizes the 38-caliber rifles and allows 45-caliber and 50-caliber loads that are unsafe.

From Long Range match observations by a number of shooters in the pits, the 38-70’s currently used by a number of marksmen that launch 370-grain bullets to about 1,400 fps with 70 grains of Swiss 1.5 black powder are safer than the larger calibers.All of those marksmen shooting the 38-70’s use less elevation between 800 and 900 yards and between 900 yards and 1,000 yards than they did when they were shooting large case capacity 45-caliber rifles. It has also been observed by those in the pits that when the X-ring is hit the 38-caliber bullets impacts are at least 2 feet higher on the back berm than the 45-caliber rifle’s bullets. Another very interested phenomenon that has been observed by a number of the pit crew is that when the crack of a 45-caliber rifle is heard the sound is no longer audible when the bullet punches through the target. When the 38-70 bullets (370 grains) are shot, the sound of the rifle crack is still audible when the bullets punch through the target. That means the 38-caliber bullets are not slowing down as much as the 45-caliber bullets and therefore must have a flatter trajectory angle at the pits.

This clearly shows that the 38-caliber bullets arrive at the pits from the 1,000-yard line just after the initial sound of the rifle crack but while the crack is still audible. The 45-caliber bullets arrive after the rifle crack has audibly disappeared.

There are several very positive benefits for the sport of LR BPCR match completion from allowing safe 38-caliber rifles to shoot LR matches. First, the heavy recoil generated by the large case capacity 45-caliber rifles keeps a number of men, women and younger folks off the LR firing-line. If the sport is to grow, and that is a good thing for the NRA, reduced recoil is very important. Also, for the people already shooting LR BPCR matches allowing them to shoot safe, lighter recoiling rifles will keep them in the sport longer as they age and can no longer tolerate the sever recoil absorbed from shooting the larger caliber rifles. I personally know of a number of competitors that have quite shooting LR BPCR matches due to the recoil they can no longer tolerate. This writer would like to see them back on the firing-line in BPCR LR matches. It is in the interest of the sport to make sure that all rifle/load combinations are safe for the pit crew as well as induce more shooters to start in the sport by allowing lighter recoiling rifles.This will also have the beneficial effect of keeping the “old guard” on the line longer.

In conclusion it should be said again that a bullet’s BC defines its trajectory angle for a given muzzle velocity.If a 340-grain, 38 caliber bullet is launched at 1,250 fps and a 650-grain, 50-caliber bullet is launched at the same MV they will both have the same trajectory angle at the pits from the 1,000-yard line as long as they are of similar design, i.e. Postell or Creedmoor. From this information it should be clear that a more rational, logical set of rules should govern the rifle/load combinations that are deemed safe for use in Longrange BPCR matches. A recommendation has been given above that is based on the science of the exterior ballistics that governs the flight of cast bullets driven by black powder out to 1,000 yards. It is the hope of this writer that a more appropriate set of rules will be considered and put into effect for the furtherance, safety and growth, of the Longrange BPCR sport.

Dan Theodore
February 28, 2005