Home Forums Hunting & Shooting Ask The Gunwriters Learning Ballistics: PBR/PBR Zero & Sectional Density

I've been reading about ballistics/playing with JBM Ballistics calculators.

I have a good bead on some things: MOA, ballistic coefficient, transonic range, range & wind adjustment. I also understand that humidity/elevation/temperature/angle affect bullet flight, but don't know to what degree or how to adjust for such shots.

However, two things still mystify me:

"Maximum PBR" and "maximum PBR zero". I assume PBR = Point Blank Range. So what is PBR and PBR zero? How is it useful in regards to hunting and paper-punching?

What is sectional density? How is it relevant to hunting and paper-punching?

Also, if there are any books or websites you'd suggest to learn about ballistics/the use of ballistic info applied to real-world shooting, I'd enjoy reading about it.

Thanks!
Jay

Jay,

Maximum PBR combines a plus or minus bullet trajectory/path over the line of sight/aiming line. I often hear the plus/minus as plus/minus 3 inches. If you were utilizing a PBR concept, you would zero at a range the bullet trajectory would not travel above 3 inches line of aim, and at max PBR range the bullet would drop below 3 inches from the line of aim.

For example, I commonly zero a 30-06 with 180 Partitions, muzzle velocity roughly 2750, at 25 yards. With this aim point the bullet is 2.8 inches above the line of aim at 125 yards, and falls below 3 inches below the line of aim just before 275 yards. So my max PBR for this configuration is 275 yards. If I wanted to shoot a big game animal within 275 yards I would expect the bullet to hit vitals, as I would aim in the middle of the chest.

Clear as mud?

As for sectional density, it's just a comparison of a bullet's weight to its diameter. Many here, myself included, believe bullet construction and placement to be of more significance than sectional density. Others would argue increased sectional density improves penetration. Probably somewhere in between.

Ever read the Berger Reloading manual? It's has excellent articles before the load data.

v/r
Joel

What he said. I'll just add that I took the data from of the significant bullet tests done here and the numbers showed that published SD has literally no correlation to penetration. Where I do think SD MAY be of some value is the case where you are comparing cup and core bullets to each other, or possibly similar design bullets to one another (accubond to accubond, etc.). When it comes to the bonded, monos and Partitions, how far they open and how well they retain weight determines penetration.

Joel made a pretty good summary of PBR.

As for sectional density, the main point to remember is that it is a mathematical assignment to differente between solid, or full metal jacket bullets a century and more ago.

To demonstrate the uselessness of it and also the "never intended" aspect relating to expanding or soft point bullets, the simple truth is that the SD of all bullets of the same weight in the same caliber is equal.

Think about that...........

Let's use .308 caliber for example:

In 220 grains, you can get a Solid (Homogenous)FMJ, HPBT, Nosler Partition, Conventional cup and core soft point.

Now you don't need a lot of experience to accept that they all could not possibly penetrate to the same degree because their construction has individual and tremendous variations in design and therefore integrity. Yet, The all have het same Sectional Density.

The reality is that there are 150gn Homogenous expanding solids that out penetrate 200gn cup and core sporting bullets of a generation ago. Less SD, less BC in many cases, but superior performance in trajectory, reduced recoil and penetration.

The rules have changed.......

I agree with what the others above have said and I think that tougher construction will generally trump SD in penetration, SD being nothing more than numbers anyway......but I think this bullet length thing might still have a place in the scheme of things.

Specifically I notice in reading the reports of those using them that the penetration of some of these very long, high BC bullets like the 180 gr 7mm Bergers seem to penetrate fairly well, and especially at the longer distances after things have slowed down a bit. My thoughts on this are not based on any personal experience because I do not use those bullets for any hunting, but rather on the reports we see from knowledgeable shooters who are using them on game.

I had trouble understanding how a thinly constructed bullet could do a decent job of penetration on larger animals but it seems that they simply are so long and heavy for caliber that they just have more weight and length to lose after impact, but enough left over to penetrate as much as they need to.....this is the only notion I can come up with that makes any sense to me as to how and why a thinly constructed bullet can get through game meat.....but I don't think it's a numbers thing...just sheer weight and length(heavy and long for caliber).I think it might also help that impact velocities are down quite a bit because the bullets are used at long range.....but what do know?

Still, I would not depend on SD numbers to tell the story, and I think that if a guy is going to use a thinly constructed bullet on anything bigger than a deer, he may be better off going to something heavy for caliber.

RobinNH
Speaking on this topic IIRC P.O.Ackley thought that the .226 Barnes QT with 125 gr.bullet very deadly on big game animals at extreme ranges because of it's high sustained velocity and tough constructed Barnes bullet. 125gr. vel. 2700fps,twist 5.5" don't if there is any thing here to compare but just mention it.
Cheer NC

northcountry that's a pretty heavy bullet for .226 cal!

I'm not sure I follow the PBR concept. So the below chart says PBR zero is 317 yards. So with a 317 yard zero, I'd be on target with +/- 5" from ~250 to ~350?





and



So if I'm comparing 2 calibers (270 Winchester and 280 Remington) that is nearly identical in every aspect (drop, wind effect, energy & velocity) with the largest variance within their sectional densities (.261 and .246 respectively); would you consider it negligible, or a factor to be considered?

Jay,

I plugged your numbers into the Hornady ballistics calculator, and it looks like if you zero at 250 yards your MPBR is 300 yards. You could probably fiddle with the numbers more, but I would doubt 25 more yards of MPBR is possible or even would make a difference. If you zero at 250 you're 2.7 inches high at 150 and 3.2 inches low at 300.

I would personally not be comfortable with +/- 5 inches over the aiming line--I'm more comfortable with +/- 3 inches. Not to say you couldn't use +/- 5 inches, but that puts your bullet 5 inches high at 150 to 175 yards, common shooting range for deer and elk. The advantage would be 50 more yards before it drops below MPBR. Since I have not shot any animal at 350 to 400 yards, this advantage doesn't matter to me, so I use +/- 3 inches.

v/r
Joel

I'm saying a Partition or TTSX of lesser weight than a cup and core bullet or bonded bullet will likely out penetrate the cup and core, assuming all else equal. Sectional density is a comparison of weights, not a predictor of penetration.

SD: Bullet Weight in pounds divided by the square of its diameter in inches.

As AGW mentioned it is the same for identical weight bullets of a given caliber since the plug in numbers are a constant, and means little with regards penetration when you factor cup/core, solid copper, etc construction.

BC: SD divided by a bullets form factor.

Therefore a high SD will end up giving you a higher BC.

Negligible... Bullet construction and expansion characteristics will have far more effect on penetration than SD numbers, and even bullet weight, when comparing those two cartridges.

Use of both in the field, on animals, will show this to be true over the long haul.

On the MPBR thing, which has been well explained, try to keep it simple to understand it.

Your line of site to the target is a straight line as you look through the scope; whereas the path of the bullet will, over distance, describe an arc. In MPBR, you are manipulating the arc in relation to your line of site so that the arc intersects the line of site at two points....one at close range(25-35 yards or so)....and the second at greater distance.

Lets take a 270 shooting a 130 gr bullet at +-3100 fps as an example, and this is how I do it myself. When properly "zeroed" the bullet exits the muzzle and "rises" (you have manipulated your sights to make it do this)to intersect my line of sight at 25-30 yards or so......the bullet will impact app 3" high, above your aiming point, at 100 yards. At 180-200 yards it will be app 3.5" high....from this point forward the bullet will start on a downward path and will cross your line of sight for the second (and last)time at app. 275-280 yards ( your "zero"). It will impact about 3" low at 300 yards....about 8" below your aiming point at 350 yards.....and about 13-14" below your aiming point at 400 yards, and so forth. You can confirm actual numbers by shooting your load at these distances.

So, from the muzzle to 300 yards, the bullet has not risen, nor fallen, any more than 3.5" above or below, your line of site.

In practical terms, and because the vitals of most big game animals are substantially larger than the mid range trajectory of your load, this means that, if done precisely, you will hit the vitals of animals with a chest hold out to a bit past 300 yards(your MPBR).

Things get more complex as we move beyond this distance where bullets from even the fastest magnums start to drop like rocks, but today you have neat stuff like LRF's and trajectory compensating devises like dots, multiple reticles, and turrets to deal with lobbing trajectories.....along with specialized high BC bullets to cheat wind and drop. But within ordinary distances at which the majority of game animals are actually killed(likely under 300 yards) the method is fast, intuitive if you are used to it, and simple to use. For me the other stuff is not of much use until distances get past the 400 yard mark.

These numbers will vary a bit depending on the actual velocity of your load and shape of your bullet but there will not be much difference between cartridges of similar case capacity tossing bullets of similar weight and shape at similar velocities....in other words you will grow old and die before you see any differences between a 270 and a 280 in trajectory or in killing effectiveness for that matter.

Some do not like this method or MPBR because the mid range trajectories of 3-3.5" is considered too high for closer range shooting but I have used it on many game animals from 20 feet to about 500 yards without much problem to speak of.