## Thursday, February 08, 2007

### What makes a pellet accurate?

by B.B. Pelletier

A question came in to Pyramyd Air's owner a few days ago. "What's the fastest speed for the most accurate pellet I can shoot?"

I say in my About ME Blogger profile that there are no stupid questions, but there ARE situations where the person doesn't know what he has asked. Therefore, an answer is either difficult to give or he wouldn't understand it when he hears it. This question is one of those.

A few parameters were missing from this question.

Caliber of airgun in question
Range at which accuracy is desired
Powerplant selected for shooting
Weather conditions
Have the pellets been sorted by weight?
Pellet shape (wadcutter, domed, pointed, novelty shape). Or, does the question also include solid "pellets" that are really bullets, and do not perform the same under similar aerodynamic situations?

All these things affect accuracy. Velocity, itself, is a variable for most of them. Without knowing it, this person has asked a question that is impossible to answer without a lecture on pellets, velocity and all the variables that affect pellet flight. In fact, this question requires a book to give a complete answer; even this blog is not enough. So let me take a very small slice of the question and try to explain the variables.

For a .177 domed diabolo pellet, what is the maximum velocity possible for accuracy at a given range of 50 yards? I ask that question because it seems to be the No. 1 concern of readers. I excluded hollowpoints because, though they may group acceptably at 25 yards, by the time they go 50 yards, they're hopelessly inaccurate.

The magic of 900 f.p.s.
No doubt you know that diabolo pellets destabilize when they get up to transonic velocities. Transonic speed is defined as a speed just below and just above the speed of sound. That's Mach 0.8 to Mach 1.2, or about 880 to 1320 f.p.s., if you accept 1100 f.p.s. as the speed of sound. Because the sound barrier is a variable itself, it is impossible to give an exact speed that will always be correct.

Look at the lower velocity - 880 f.p.s. That's the lower limit of transonic flight. Transonic flight is a speed at which the airflow over the body of a projectile is moving at supersonic speeds at some places and subsonic speeds at others. A barrier wave is created at transonic speed, and that wave causes flutter on a shape not designed to handle it. That flutter causes the projectile to wobble, which causes destabilization. Like it or not, the transonic range is the worst range at which to shoot a diabolo pellet. And because of its shape, a diabolo pellet slows down so rapidly when accelerated to supersonic speed that it's into the transonic region soon after leaving the muzzle - even when the muzzle velocity exceeds 1400 f.p.s.! So, shooting faster than 900 f.p.s., is bad with domed diabolos all the time, and worse on days when the sound barrier is below 1100 f.p.s., because the pellet remains transonic for a longer period of time.

Food for thought
If you're with me to this point, here is how changing one other variable changes the entire question. Substitute a solid pellet - a bullet - for a domed diabolo and everything I have said changes. A solid bullet can go faster than the sound barrier without a problem. In fact, the new Piledriver pellets are shaped correctly to do just that. Their boattail base and rebated skirt lower drag significantly. Theoretically, solid pellets could be accurate at supersonic speeds.

This is the .177 caliber Piledriver pellet, looking at the tail end. It weighs 20.7 grains. This pellet is too heavy and too low-drag to properly stabilize at normal air rifle speeds, and no commonly available rifle can shoot it fast enough for accuracy at long range.

HOWEVER - and this is a BIG however - no pellet rifle currently made has a twist rate fast enough to properly stabilize a Piledriver pellet at the velocities it can be shot, so at 50 yards the accuracy isn't there! Unless the Piledriver is accelerated to around 1100 f.p.s., it will wobble on its axis and the groups will open, the farther it moves from the muzzle. Shooting this "pellet" at 600-800 f.p.s. would be futile, if the goal is accuracy at 50 yards. The proof of this, if you don't have any Piledrivers to shoot, is to try to group a .22 CB cap at 50 yards. The 29-grain lead bullet that groups well at 25 yards will spread out at 50 yards, because the standard 1:16" twist of a .22 rimfire barrel (which is identical to the twist rate of most airgun barrels) is too slow to stabilize a 29-grain bullet when it moves that slow.

Like I said at the start of this post, some questions ask more than was intended. This has been an attempt to explain a portion of the answer to the question we looked at here.

At February 08, 2007 9:42 AM,  Vince said...

How badly do supersonic and transonic speed effect a round ball?

At February 08, 2007 9:50 AM,  Anonymous said...

B.B

Quick question, if you don't mind. I was wondering if I site my iron sights on a benjamin 392 at lets say 25-30 feet. Would still hold a bulls eye "quarter" at 25 or 35 yards? Or does it have to be sighted in at those ranges. I would like to be dead on from 35 yards down to 20 feet. But I'm new to this and the pellet/bb combo I had was not something that I shot at targets with. I was impressed with the pellet cluster at 25 feet and I could really enjoy target shooting. It's a great feeling to hit a quarter sized bulls eye at 30 feet for me. I can't imagine doing it at 105ft "35 yards". That would give me a great feeling of accomplishment. I really enjoy your post and your help during the beginning of my renewly found hobby. Thanks, Nathan

At February 08, 2007 1:01 PM,  Don said...

B.B. This is off of the topic: I just ordered the RWS 850 .22 Cal Combo from Pyramyd and I was wondering if there is a way to bulk fill the gun with CO2? I bulk fill my Tau, Drulov, Alfa Proj, and my AR2078 (Chinese) guns. I know that you posted three excellent reviews on the 850 but I couldn't find anything on "possible" bulk filling. Perhaps I couldn't find anything because there is no way (now) to safely do it. Thanks.

At February 08, 2007 1:27 PM,  B.B. Pelletier said...

Vince,

Round balls don't seem to be affected by transonic flutter or supersonic speed, but they do slow down almost as fast as diabolo pellets.

B.B.

At February 08, 2007 1:28 PM,  B.B. Pelletier said...

There is no way to be sighted from 20 feet to 35 yards with an airgun using the SAME sight setting.

If you are sighted to hit at 35 yards, you will be low at 20 feet.

B.B.

At February 08, 2007 1:29 PM,  Anonymous said...

B.B. Off topic: Just what is a weaver base and how is it different from other scope mounts?

At February 08, 2007 1:30 PM,  B.B. Pelletier said...

Don,

There is no mainstream way to bulk-fill an 850, but I am sure there are numerous ways to do it. Check out the airgun forums

B.B.

At February 08, 2007 1:50 PM,  B.B. Pelletier said...

Weaver base,

I can't believe I have not blogged Weaver bases yet, but I will.

Thank you!

B.B.

At February 08, 2007 4:30 PM,  dm20 said...

if you were to take number from the top of your head, how many weapons do you think you own, powderburners and airguns alike? i had no idea you had a garand, of all things!

At February 08, 2007 6:46 PM,  Anonymous said...

hi bb,
i am looking for a good field target gun. i read your posting on what is a field target and followed a link and another and found out how to make my own field target. ive made 5 so far. so i need a better gun. this is a futuristic thought because i dont have the finances. i would love a pcp but they are a little ouut of my price range. here are some of my cocern areas,
1 price (id like to keep the gun under \$350)
2 accuracy
3 scopability
4 trigger
5 powerplant
also think you could recomend a good variable power scope?

Field Targetier

At February 08, 2007 6:59 PM,  Jensen said...

Good post B.B.

Perhaps you should have pointed out the resources we have here on the web that would better explain what exactly effects pellet accuracy. I read up on most of the articals at www.AirGunExpress.com . I'd like to share these links with those who haven't read them yet, because they are an absolute WEALTH of information. =)

These articals are all intertwined when you are considering the accuracy of your pellet. They answer individual questions, sure - but to better understand what effects your pellets trajectory, you should read them all.

Air Gun Accuracy

What is Muzzle Energy

Airgun Calibers

Pellets vs. Round Balls

And finaly,

VELOCITY AND PELLETS!

Hope someone takes alot away from these. I sure did!

~Jensen

At February 08, 2007 7:00 PM,  Anonymous said...

Oops, sorry about that. Looks like I forgot to close up that first link. =P

At February 08, 2007 7:59 PM,  Anonymous said...

witch would be better for hunting at long range as far as getting there and accuracy at long range say 75 yds or so a .22 or .25 career shooting eun jins??? because ive decided on the gun but now im stuck on the cal. thanx for any advice ,zach (ps your blog is the greatest thing ever)

At February 09, 2007 5:51 AM,  B.B. Pelletier said...

dm20,

Yes, my Garand is a post-Korean H&R, and it's accurized just a bit. I love it because I drilled with one for four years and ran the bayonet course with one.

B.B.

At February 09, 2007 7:51 AM,  B.B. Pelletier said...

The pellet with the best ballistic coefficient will be the best long range pellet. For the Career, it will be a .22.

B.B.

At February 09, 2007 7:56 AM,  B.B. Pelletier said...

Field Targetier,

As long as the trigger is number 5, I'd go with the BAM B40 in .177. Being a close copy of the TX200, it is very scopeable.

If you aren'y going PCP, spring-piston is the only other choice. CO2 is too temperature-dependant.

For field target you need a scope with some power. I suggest the Leapers 6-24.

B.B.

At February 09, 2007 9:28 AM,  Anonymous said...

BB,

Very close to the pellet is the pellet probe. How does a pellet probe affects the accuracy of pellets? What is a good pellet probe, pin or cylinder?

Dave

At February 09, 2007 9:53 AM,  B.B. Pelletier said...

Dave,

I like a pin-type probe, but only if it fits the pellets I want to use. If not, a large transfer port and the wrong pin can tip a pellet and shave one side of the pellet's nose. That's not good.

B.B.

At February 09, 2007 1:31 PM,  Anonymous said...

ok nice
is that ambidexterous? i forgt to mention im left eye dominant.

Field Targetier

At February 09, 2007 3:05 PM,  B.B. Pelletier said...

Field Targetier,

Well, it LOOKS ambidextrous. I'd call to check for sure.

B.B.

At February 11, 2007 5:55 PM,  Anonymous said...

B.B.,

Can you please do a blog on pellet probes?

Thanks.

Dave

At February 11, 2007 9:09 PM,  majorkonig said...

Hi BB,
Can you do a post on terminal ballistics? Low vel. heavy shots versus high vel. light shots ... I know, sounds like firearms, but I am sure it has implications for the airgun hunting community at least ....

At February 12, 2007 7:37 AM,  B.B. Pelletier said...

Dave,

A blog on pellet probes? I'm afraid it would be very basic. Let me give it some thought and maybe you can tell me what it is you'd like to know.

What do you want to do with the information?

B.B.

At February 12, 2007 7:48 AM,  B.B. Pelletier said...

majorkonig,

FSI has published an entire book of ballistics tables. Maybe you really want that?

I'm not sure what you are looking for. can you please clarify?

B.B.

At February 12, 2007 10:33 AM,  majorkonig said...

Well BB, after reading a lot abt heavy bullet ballistics and their advantages, my concepts were literally blown apart when I saw wht a 0.20 high velocity round was capable of. It almost blows apart midsize varmints. The airgun connection is with bigbore airguns. Quackenbush is developing a .58 cal beast that is putting out 800 fpe. Wondering if hiking up energy levels by just making bigger and bigger calibers is really worth it ... No disrespect to Dennis intended, just my doubt ...

At February 12, 2007 10:38 AM,  B.B. Pelletier said...

majorkonig,

I think you will be very interested in tomorrow's post!

As for the velocity vs bullet weight controversy - that's Roy Weatherby going up against Elmer Keith. I tend to be in the Keith camp, but I still get surprised by the high-velocity crowd, so I guess I don't know very much, either.

B.B.

At February 12, 2007 7:55 PM,  Anonymous said...

For those who like experiment, and have nice piles of snow. Try shooting into it, level with the ground, it stops bullets (ie 38 special)without damage of any kind, and in a unbelievably short distance. Tryed it with high power rifles at longer range, same result. Try it.

At February 13, 2007 10:39 AM,  majorkonig said...

Hi BB,
Thanks for the reply. Yup I like comparative tests. That was another great post.
Abt the high vel crowd, yes, its strange that a tiny high vel bullet punches through so well, when every ounce of common sense says that at those blistering speeds, the bullet would just fragment on impact. Ballistics is fascinating but I dont like the fact that I dont quite understand this particular phenomenon. I quess a VERY detailed course in physics is what I need !!!

At February 14, 2007 12:41 AM,  Anonymous said...

is the theoben eliminator capable of shooting the piledrivers accurately?and towhat distance?
Thank you.

At February 14, 2007 8:36 AM,  B.B. Pelletier said...

Of all spring guns, the Eliminator should be capable of shooting this pellet, but I'm just guessing. As for how far, I don't know.

B.B.

At February 15, 2007 1:39 AM,  Anonymous said...

BB,

I think that a pellet probe has some influence in the proper seating of the pellet in the chamber.

I do not have it but I suspect the pin type probe might some advantages re accuracy over the tube type. Since my rifle has the tube, I am thinking of cutting the tube part and welding on a pin type probe.

I look forward to your post on probes someday, if ever.

Dave

At February 15, 2007 8:52 AM,  B.B. Pelletier said...

Dave,

If the probe you are thinking of cutting is hollow like the bolt on a Benjamin 130, you will ruin your rifle! A hollow probe is actually part of the air transfer port system.

If the probe is solid like on a Crosman 2240, then yes, a thinner pin might improve airflow, but welding it out because the bolt is made of brass.

B.B.

At February 24, 2007 12:23 AM,  Anonymous said...

B.B.

Yes sir my probe is the hollow type, and you are right it is a part of the transfer port. My concern is that, since the probe's body do not make a perfect slide(there is too much play) to seat a pellet each time, it may contribute to my wandering grouping somehow. Besides, since the air rushes through the hollow probe, doesnt it consist of a constriction in air flow?

what is the best flow to produce accurate result, laminar or turbulent or does it matter?

Dave

At February 24, 2007 7:45 AM,  B.B. Pelletier said...

Dave,

A hollow probe means your gun is a BB gun, with a smoothbored barrel. There is no way to increase the accuracy on that model.

Leave it alone and get a pistol with a rifled barrel if you want better accuracy.

One thing you could experiment with is shooting 4.4mm lead balls. They are one thousandth larger than BBs and may improve accuracy a little.

Go to John Groenewold for them. PO Box 830, Mundelein, IL 60060-0830, (847) 566-2365

B.B.

At April 26, 2007 9:58 PM,  tarsch@hotmail.com said...

I'm running a Daisy 22SG, and I'm having a real problem trying to keep shots within a 3 X 5 Card area at 40 to 50 feet. I've tried with 3 different (similarly weighted) ammuntions with and without a scope, while the pattern randomly switches. Is my ammo too light (too fast), or my barrel dirty or worn. I couldn't see the transonic problems affecting a rifle that advertises only around 600 F.P.S. Will test with fewer pumps, but how would a slower speed traded for better accuracy affect hunting? tarsch@hotmail.com

At April 27, 2007 6:20 AM,  B.B. Pelletier said...

tarsch,

Your problem isn't velocity or pellets. I think you may have a loose barrel! Several readers have complained of the same problem and that's what it turned out to be.

Please safely try to wobble your barrel and see if it moves. If it does, and ESPECIALLY if you are using a scope or dot sight, that is the problem.

The solution seems to be having Daisy repair the gun.

As for pellets in the Daisy, I got good results with Crosman Premiers. Jim House likes Daisy pointed pellets and Gamo Match wadcutters.

B.B.

At May 15, 2007 6:09 PM,  Anonymous said...

BB,

I heard that pellets over 900 fps are not accurate and that you have to lube them, I was planning on buying an RWS 48 for field target, is it a good choice? if not, what would be a good choice keeping in mind a price similar to the 48?
Would you recomend any other? could you tell me waht pellets would work best for both, field and short distance paper targets?

Alex

At May 16, 2007 7:45 AM,  B.B. Pelletier said...

Alex,

Many who use the 48 for field target detune it and shoot Crosman Premier 7.9-grain pellets. I would also experiment with JSB pellets weighing under 8.5 grains. Domed, of course, for FT.

As an alternative gun, I think the BAM B40 is good. The only fault I can see is the trigger is too light.

B.B.

At February 01, 2010 1:16 PM,  Anonymous said...

real strange,..my BSA Scorpion rifle groups the .177 PD into 1/2" at well beyond 55yds and then at 12ft/lb muzzle energy.

At February 01, 2010 3:57 PM,  FRED said...

BSA Scorpion,

very good shooting! However, very few people go to these old blogs and review comments. There are a select core of us who volunteer to do that to provide any answers but more so, to ask you to join the Blog on a regular basis and go to the current blog.

Posting comments there will be sure to generate numerous responses and questions (such as what type pellets you used, what scope are you using and so on).