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deadrats · Member

Does anyone have a good sense of what pellet weight does to velocity in actual numbers. I know some of you chrono your guns with various pellets so maybe someone can give me an estimate. If i have a gun that shoots 177 pellets at 425 (assuming manufacturer velocity specs are arrived at with whatever is considered average pellet weight) how much velocity would you lose for each extra grain you add ? If 425 is a 7 grain for example, where do you think the velocity would be with a 10 grain?

rsterne · Moderator

A 7 gr. pellet at 425 fps is 2.8 FPE....

A 10 gr. pellet at 2.8 FPE would be travelling 355 fps....

There is no "law of pellet guns" that states this will occur in real life, however....

Bob

deadrats · Member

but thats not exactly what i'm asking. What i mean is if a gun does 435 FPS with say a 7 gr, what would the velocity drop to if say you put a 10 gr in it? better yet, how many FPS loss for every grain approximately. I don't expect a lot of people to know this, but i would imagine thos who chrono pellets in various sizes thru thier guns have probably come to be able to estimate in thier head approximate velocity drop for a given weight over another.

Slavia · Moderator

rsterne · Moderator

The formulas that slavia gave you are EXACTLY the way I got the numbers I gave you.... If you wanted a different set of numbers why didn't you ask for those?....

There is no "set number of fps" change for a 1 grain change in pellet weight.... Obviously the change will be greater for a gun shooting 800 fps than for one shooting 700 fps.... You can reduce the number of steps if you only want to compare two pellet weights by doing the following with a calculator (in this order):

Enter the velocity with pellet #1
Square it (multiply it by itself)
Multiply the answer by the weight of pellet #1
Divide that answer by the weight of pellet #2
Take the square root of that answer- this will be an ESTIMATE of the velocity of pellet #2

EG: Pellet #1 is 7 gr. @ 435 fps, Pellet #2 is 10 gr.

435 squared is 189,225
multiply by 7 gives 1,324,575
divide by 10 gives 132,457
square root of that gives 364 fps

That is your ESTIMATE for the velocity of a 10 gr. pellet if the gun fires a 7 gr. pellet at 435 fps....

HTHs

Bob

deadrats · Member

rsterne · Moderator

The manufacturers often (but not always) use the lightest pellet available.... This is particularly true of Gamo (who sell the Raptor lightweight pellet).... and less true for the older guns such as the Crosman 22XX and 13XX series (that were around before the extremely lightweight pellets became available).... You can pretty much take the published velocity figures with a grain of salt.... The reason is that "Speed Sells"....

Most guns advertised at 1000 fps in .177 and 800 fps in .22 will actually shoot ~850 fps in .177 with a 7.9 gr. pellet and ~680 fps in .22 with a 14.3 gr. pellet.... Those pellet weighs (7.9 gr. and 14.3 gr) are usually considered by the airgun community as the "standard weight" for .177 and .22 cal respectively....

There are comprehensive tests of a lot of airguns on the Straight Shooters website:

http://www.straightshooters.com/navagationpages/ssproductsandfeatures.html

Select the type of gun, then click through to the model you are interested in.... Once on the page for that airgun, click on "Our Take" and then on "Velocity Test Results".... You can pretty much rely on that information as being representative of the performance of that gun in the real world....

Bob

deadrats · Member

fantastic link, thanks !

JustMatt · Member

rsterne · Moderator

The constant 450240 is derived in the following way:

7000 grains per lb. x 32.16 ft/sec^2 (weight to mass conversion @1G) x 2 = 450240

The formula for energy (FPE) is E = 1/2 MV^2 where M is the mass is slugs and V is the velocity is in ft/sec.... Where the weight W is in grains, the formula becomes:

E = 1/2 x W/7000 x 1/32.16 x V^2
or E = W x V^2 / 450240

HTHs....

Bob

JustMatt · Member

Thanks for the explanation, I figured I had to be doing something right to get the same values over and over.

donwalk · Veteran Member

i did some chrono test with my RWS 52 in .177 a few days ago for the most accurate pellet to use in this rifle.

10.5/10.0 grain and 8.3/8.0 grain pellets

125fps difference

only one to two fpe difference in all of them

10.5/10.0 grain pellets were highly accurate

8.3/8.0 grain pellets not so accurate

my tests showed the heavy pellets to be most accurate. (from my particular rifle)

avg 12 fpe for the 8.0/8.3 grain pellets

avg 11 fpe for the 10.0/10.5 grain pellets

i tested seven different style/name brand pellets shooting ten shots per test group for a total of seventy shots.

i used open sights and a mechanical front rest on a bench at a 1.25", black, bulls-eye, hand drawn, target at a measured seventeen yards

smallest group: less than one inch (Beeman Silver Jets)

biggest group: over two inches (Beeman Silver Bears)

the fpe looks a little low from my rifle, maybe, but it's a model 52 i purchased new in 1988-1989 so it may be in need of 'tuning' by now.

donwalk · Veteran Member

an interesting caveat that's related to this topic:

i saw on the news recently an article that was covering the trial of a border patrolman accused of shooting an illegal alien because the illegal threw a stone at him.

the FBI conducted tests involving the throwing of stones as it relates to weapons and found that:

a one (1) pound stone thrown, by/from a human hand, at 180 fps hits as hard as a 9mm slug fired from a pistol.

i don't recall the exact fpe quoted, but there was only a few fpe difference.

this re-enforces "more weight=more energy".

Lipoly · Veteran Member