The AirGun Guild

General Category => Ask Bob => Topic started by: oldpro on March 07, 2016, 11:57:05 AM

Title: Pellets vs. bullets
Post by: oldpro on March 07, 2016, 11:57:05 AM
 Ive always wondered why Pellets of the same weight and bore as slugs do not carry energy as well down range is it simply the BC or is there something else to it? thank you Mr. Sterne for taking the time to answer all these questions im sure you will get.
Title: Re: Pellets vs. bullets
Post by: rsterne on March 07, 2016, 03:01:52 PM
The Ballistics Coefficient of a projectile is a function of the Sectional Density and the Form Factor (drag).... BC = SD / FF to be exact.... If you look at a typical bullet, say the G1 Drag Model, for example, by definition it has an SD of 1.000 and a FF of 1.000, so the BC is 1.000.... The FF for a sphere (roundball) for example, is 1.55, so if you had a sphere with an SD of 1.000, it would only have a BC of 1.000 / 1.55 = 0.645.... Pellets, like a sphere, have more drag, and hence a higher FF (which is a measure of the drag) compared to a bullet.... In fact, they are pretty close to the drag of that sphere, for a round-nose pellet like a JSB Exact, even worse for a wadcutter, they might have a FF of 2.0 or even more....

In your question, you specified that the bore and weight are the same, so they have the same SD.... The SD is the weight (in lbs.) divided by the caliber^2.... For a 25.4 gr JSB King, .25 cal the SD is (25.4 / 7000) / 0.25^2 = 0.058.... If the FF is 1.5, the BC would be 0.058 / 1.5 = 0.039, which is pretty close, so that was a good guess for the FF.... If you had a bullet the same weight, but less drag and a FF of 1.0.... the BC would be 0.058 / 1 = 0.058.... That higher BC is an indication that it will lose velocity less quickly as it goes downrange, and that it will also be less affected by the wind.... so the bullet will have a flatter trajectory, higher retained FPE, and less wind drift than the pellet.... all because its shape has less drag....

Bob
Title: Re: Pellets vs. bullets
Post by: oldpro on March 07, 2016, 10:11:00 PM
 Ok thats about what I thought No magic bullet just plain old BC. Is there a pellet design with superior BC but still accurate?
Title: Re: Pellets vs. bullets
Post by: rsterne on March 08, 2016, 08:28:33 AM
Darned if I know.... *LOL*.... Accuracy is a funny thing.... It has a lot to do with bullet fit in the bore, and also matching the bullet (length primarily) to the twist rate.... Boattails act like they are longer than they are.... which is why they have less drag.... but also why they need a faster twist.... If the twist is too slow the bullet becomes unstable and tumbles.... This is called Static Instability.... If the twist rate is too fast, the bullet tends to spiral, generally getting worse the further it goes downrange.... This is called Dynamic Instability....

The biggest factor in BC is still weight (SD), though.... It pretty much trumps shape in any given caliber....

Bob
Title: Re: Pellets vs. bullets
Post by: Habanero69er on March 08, 2016, 09:36:19 AM
Bob, what would you say is the proper twist rate for say a 45-50gr slug in a .25, 16" barrel? I'm looking for ammo for my Sumatra carbine which has a 1:18 twist. If 45-50gr isn't appropriate for that twist rate, what would you suggest?
Title: Re: Pellets vs. bullets
Post by: rsterne on March 08, 2016, 01:32:33 PM
That should be OK.... If it is my 51 gr. BBT, a 14-16" twist would be better, but for flat base of those weights, or for my 41 gr. BBT, it should be perfect.... I believe Manny used my 51 gr. BBT in an 18" twist Korean barrel and loved them....

Bob
Title: Re: Pellets vs. bullets
Post by: Habanero69er on March 08, 2016, 06:08:45 PM
Where can I buy some of the 41g BBTs?
Title: Re: Pellets vs. bullets
Post by: rsterne on March 08, 2016, 07:32:49 PM
The molds are currently in Group Buy at NOE.... http://noebulletmolds.com/smf/index.php/topic,1319.0.html

Bob
Title: Re: Pellets vs. bullets
Post by: Habanero69er on March 08, 2016, 07:57:39 PM
Yeah, I'm not casting though. I need to find someone who does.
Title: Re: Pellets vs. bullets
Post by: scottyhazzard on April 09, 2016, 05:55:24 PM
Try contacting http://nielsenspecialtyammo.com/
I know he is working toward having the .25 BBT, however I don't yet see them up on his site. I hope when he does get them up and running that they look more like the .35 cal version than the .30 cal version. I don't know why but I really like the look of the BBT. I want them and also don't plan on casting at this point in time.
Title: Re: Pellets vs. bullets
Post by: Bob La Londe on April 09, 2016, 06:50:03 PM
Yeah, I'm not casting though. I need to find someone who does.

Casting is pretty darn easy one you get the knack.  Buy your lead from Roto Metals and you will get exactly what you expect.  Or gamble on Fleabay lead and who knows what you will get. 

Title: Re: Pellets vs. bullets
Post by: nielsenammo on April 09, 2016, 08:55:12 PM
Those BBT molds for .25 have never been offered to sale.  They had some delay right when the mold was supposed to be made and then nothing ever happened.  For me I would buy the mold from NOE and see how it shoots.  If it shoots I would then have my custom mold maker make 6 molds of that design to go on my casting machine.  I don't hand cast any of my bullets except test bullets.  All the cast bullets on my site are made from a machine set to make about 3,000 bullets per hour.

Lead is so cheap on a personal level I don't know why you would not just buy certified lead and get the best consistency. You go through the learning curve and buy all the stuff so to save pennies makes no sense to me.
Title: Re: Pellets vs. bullets
Post by: Monkeydad1969 on April 10, 2016, 07:30:31 AM
Lead is so cheap on a personal level I don't know why you would not just buy certified lead and get the best consistency. You go through the learning curve and buy all the stuff so to save pennies makes no sense to me.

Because you're rich!  You pompous arse!   ;D
Title: Re: Pellets vs. bullets
Post by: nielsenammo on April 10, 2016, 08:12:52 AM
When you buy the cheap lead you save say $1 per pound and depending on the grain bullet say that makes 100 bullets, you save $0.01 per bullet what was only costing say 5 cents to make anyway.  The pain with casting is just the time to do it, unless you shoot a lot I don't think it is worth the time.  If it is worth your time, why step over pennies is all.
Title: Re: Pellets vs. bullets
Post by: Bob La Londe on April 10, 2016, 08:42:09 AM
"WHEN ASKED"

I always tell my lure mold customers to buy quality lead from Roto Metals, and weigh in bits of their hard lead alloy to get what they want if they want something harder.  That way they get good consistent pours.  The problem is people don't listen, and buy all kinds of salvage lead.  I pretty much use 95:5 Lead:Antimony for everything myself after much experience with salvage.  I do have some of Roto's hard lead alloy, but I don't make bullets so haven't used it.  I started to say I don't cast bullets at all, but I do have an old .36 ball mold around somewhere from when I shot cap and ball as a kid.  I used salvage lead, and probably got 40% bad bullets.

I have three electric lead pots and a handle cast iron pour pot I use on a propane turbo burner just for testing customer molds before I ship them out.  The problem with Fleabay and salvage lead is a lot of it has huge amounts of other metals in it.  I hesitate to even call some of it alloy because when you put it in a lead pot cranked to high it melts out the lead leaving a skeleton of the other stuff.  Even on my propane turbo burner. 

A few years ago I sent a buddy of mine a bunch of soft plastics I make for my own use.  He paid me back with several hundred pounds of his salvage lead melted into muffin tins, packed up in small plywood crates, and shipped to me in large PM FRBs.  He had about half of it marked as "hard" with a magic marker.  After about the third hard ingot that came out mostly as a skeleton of a muffin I decided it the free lead was a false economy.  I decided to melt everything down and pour it in ingot trays.  The skeletons went in the trash, and the ingots that came out heavily wrinkled on the sides went in the trash right behind them.  I actually saved about half of what he sent me by weight.  Less than half by volume.  I spent most of a day doing that.  Before I  sorted the lead by melting I wasted a lot of time testing customer molds with bad lead and thinking I had screwed upt he mold somehow.  Some of those molds went back on the mill two or three times tweaking them to pour easier.  I'ld sure like to know what he used to melt the stuff to begin with. 

Trust me, even if you only value your time at minimum wage its can be a false economy to use a lot of salvage lead from unknown sources. 

Title: Re: Pellets vs. bullets
Post by: Alan on April 10, 2016, 09:23:12 AM
No one asked, but....

Lead is the most recycled metal on the planet—very close to 100% in the EU, and a few percentage points less in the USA. About 90% of the lead in new batteries comes from recycling, with the remainder other metals to adjust hardness, porosity, etc. So, when we say "the rest goes in the trash", I hope it goes to recycle!

I save the left over lead in my bullet trap, and remelt it in a soldier pot. I do it outdoors, because a lot of lead which comes from foreign sources has a fair percentage of arsenic. Which, by the way, is a good reason to use virgin lead sources. Then off to the collection station about once a year.