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Jaxom · Senior Member

I've read along with just about every thread on here about increasing powr for the 22xx and discovery. I've tried my best to understand some of the more in depth threads that go into extensive detials.

What I've understood so far is this. More power equates to fewer shots. This makes perfecte sense because in order to have more power you have to use more high pressure air. The more air you use per shot, the fewer shots you can take before needing to refill again. What I still haven't grasped yet and am hoping to clarify is how does extra power corrilates to acccuracy.

(this topic can apply to both rifles and pistols, so that's why I'm posting here in general)

For my example, I take my Discover. Install a Boss Discover valve, which has a larger volume. I enlarge the port on the guage block, and install the heaviest hammer spring recommeded to be used in this application. I also widen out the transfer port and the barrel port as well.

Given these mod's and such, I should in theory have the highest flow possible. Of course as I stated eariler I'll also have the least amount of shots per fill.

But how is this going to affect pellets down range I'm wondering? Again for the sake of this example I'll be using 21gr Kodiak's. They're not the heaviest .22 pellet but not the lightest either. Being domed they should give good flight characterisics. I've had good luck with these pellets in my 2300, being rather accurate. But happens when I'm pushing them a 2x the speed of what my 2300 can do using just C02?

Again, I do understand this much, dialing back the pressure a bit by not filling the gas tube as much, dialing the hammer spring back as well, I'll get more consistant shot groups in both fps and fpe.

What I'm trying to figure out is just how far you can push a discover keeping tighter shot groups at range. The cost of filling my scuba tank or having to carry a pony tank is nominal from my point of view.

Alot of this has to do with my opinion that one should practice, practice, and do even more practicing. This way once in the field, if my discovery only get's (for an arbitrary number) 30 shots out to 100 yards, I know full well I will hit what I'm aiming at. These hypothetical 30 potential shots should equate to 25-30 rabbits for my stew pot.

That's the whole point of this question. Pushing the discovery as far as I can and not have shot groups look like I was using a shotgun.

This again can be applied to pistols as well. Like my 2300. Granted since I will be replacing the stock gas tube with a cut down discovery tube, I won't have as much volume to work with. It would hardly make sense to go to the max with this project. Yet, pushing that as far as I can at the same time.

Thanks,

Jax

rsterne · Moderator

The velocity itself should not have any negative effect on the accuracy.... and it might even have a positive effect.... Just like any air rifle, pellet choice is key for the combination you have.... along with things like a good barrel crown, good shot-to-shot consistency, etc.... Also, I would stay below roughly Mach 0.8 (~900 fps) to avoid negative transonic effects on the Ballistics Coefficient.... and below about 1000-1050 fps to make sure there are no shock waves forming on any portion of the pellet which might cause it to wobble or tumble....

Any barrel flexes during the shot.... and the thin Crosman barrels are more flexible than most.... This really doesn't matter providing you match the velocity/pellet combination to the barrel harmonics.... I had a .222 Remington (burner) that when I was working up handloads you could watch the group size change and the POI walk around the target at 100 yards.... The POI would track in an oval shape about 2" wide and 5" long on an angle from about 1:00 o'clock to 7:00 o'clock.... The tightest groups would occur at the ends of the oval when the barrel was reversing direction.... and the worst groups in the middle of the long sides of the oval.... The pattern was consistent with various powders and bullets.... but of course some combinations were much tighter grouping than others.... The best groups occurred at velocities near maximum and about 400 fps lower.... but at a totally different POI.... It took a 400 fps difference in velocity for the barrel to move from one end of the oval to the other as it whipped along in it's oval pattern....

Just to give you an idea.... I have a 2260 HPA conversion that will give me ~50 shots at 950 fps with JSB 18 gr. Exact Heavy pellets and put 5 shots into a dime a 50 yards.... It will shoot the same group size at 725 fps as well.... and also at ~580 fps (14.3 gr.) on CO2.... Virtually the same accuracy at 11 FPE, 21 FPE, and 36 FPE.... That good enough?.... Mr. Green

Bob

packrat · Senior Member

That's obviously the one that you said was the most accurate gun you'd ever shot Mr. Green

Jaxom · Senior Member

The Guru speaks!! Thanks Bob! I'm beginning to see the light. If I read that right and you remove varibles like a poor crown, it's almost as if you are starting with a new ag. Even if you have 1000's of rounds of various pellets through it, by doing even a modest tune or tweak you need to find the new "sweet spot" or the like. Of course by that you'll have a better idea of what types of pellets your particular ag likes and dislikes.

Now I do understand the latter half of this statement....

rsterne · Moderator

Once you go above Mach 0.8 you enter the "Transonic Region".... The Ballistics Coefficient takes a nosedive over ~900 fps.... The experimenting I have done on the JSB Exact series (an excellent, round nose pellet) shows that the BC drops as the velocity approaches this region and REALLY drops over 900 fps.... The heavier the pellet, the higher the velocity where this occurs, but only by a bit....

14.3 gr. Exact Express are losing BC after about 800 fps.... 15.9 gr. Exacts at about 850 fps.... and 18.1 gr. Exact Heavies by about 900 fps.... The loss is HUGE.... As an example, the 18.1 gr. Heavy drops from 1014 fps to 983 fps (a loss of 31 fps and 6% of it's energy) in the first five yards.... A wadcutter like the RWS Hobby that is just subsonic loses about 96 fps (and 16% of it's energy) in that same 5 yards....

Here are some comparisons with the 18.1 gr. JSB Heavy at different MVs:

1014 fps MV down to 875 fps at 25 yards - loss of 14% - BC = 0.022
967 fps MV down to 860 fps at 25 yards - loss of 11% - BC = 0.027
899 fps MV down to 815 fps at 25 yards - loss of 9% - BC = 0.032
819 fps MV down to 754 fps at 25 yards - loss of 8% - BC = 0.038
710 fps MV down to 652 fps at 25 yards - loss of 8% - BC = 0.036

As you can see, trying to push a pellet faster than about 900 fps becomes very inefficient.... Not only does it take a lot more air to get the last bit of velocity.... you end up losing a large part of what you gained in the first few yards.... That is why I don't usually push pellets past about 900 fps.... not worth the air to do it.... Just another reason for using heavy pellets in the more powerful airguns....

HTHs....

Bob

Jaxom · Senior Member

So your advice then is after doing all the mods, tunes, ect. Get a chrono, so, by using the hammer spring adjuster, you can dial it down till just below 900fps. No matter what type of pellet I'm using.

Okay, that's what exactilly what I was looking for. My only last concern would be at what range would you say there's still enough energy to make an accurate kill? (without having to go with an extreme hold over)

I know that in my case where I'm looking at rabbits, these aren't the toughest of crittters to dispatch. But I don't want to take a shot outside the range my rifle (and my ablities) are capable of.

rsterne · Moderator

You can taylor the power by changing pellet weight.... The 900 fps is a rough guide, lighter pellets will start to lose BC below that value and really heavy pellets may go a bit beyond.... In something like a Condor (60 FPE) you'll end up using something like EunJins at ~1000 fps....

Once I get beyond about 800 fps with 14.3 gr pellets (~20 FPE) I start looking at heavier ones (16 gr. JSB Exacts).... Once the gun will drive those at over about 850 fps (~25 FPE) then I try the 18.1 gr. Heavies.... If the gun will spit those out at more than 900 fps (~32 FPE), then I look at 21 gr. Kodiaks.... I haven't had a gun that was powerful enough to require me to go heavier.... and I don't know what the upper speed limit is for Kodiaks as I haven't done enough experimenting.... I can tell you that their BC is still 0.034 at 904 fps.... which is down from the peak but still pretty darn good.... I'm guessing their upper limit would be maybe 950 fps (~42 FPE).... Beyond that energy, you are in EunJin country....

It is possible to use pellets at velocities higher than what I specified above, of course.... IMO, however, the gains in efficiency from going to a heavier pellet outweigh the extra muzzle velocity of the lighter one because you lose so much of it in the first few yards anyway.... Ultimately, of course, you have to use the most accurate pellet in your rifle.... but if you are shooting at long range I don't think there is much point in looking at anything other than round nose pellets with a good BC.... The losses are just too great....

Incidently, an 18.1 gr. JSB Heavy at 900 fps has ~32 FPE at the muzzle.... ~23 FPE at 50 yards.... ~20 FPE at 75 yards.... and ~17 FPE at 100 yards.... If you sight it in at 45 yards, it will be ~0.6" high at 30 yards.... ~0.5" low at 50 yards.... ~5.2" low at 75 yards.... and ~14.3" low at 100 yards.... If you have a MilDot scope (with accurate spacing) you will be 1 dot low at ~62 yards, 2 dots low at ~76 yards, 3 dots low at ~89 yards, and 4 dots low at ~100 yards....

For rabbits, if you can place the shot well enough, that would be tons of power out to 100 yards or a bit more.... The problem is, that in a Disco at those power levels (~32 FPE).... you will likely get only about 15 good shots per fill.... Drop the power to ~25 FPE and you should be able to bet ~20 good shots per fill.... Stock, you will be at ~25 good shots.... A JSB 15.9 gr. Exact from a stock Disco will still deliver over 10 FPE at 100 yards....

HTHs....

Bob

Mark5043 · Silver Status Member

sniper · Silver Status Member

thanks for the explanation bob. that's what my understanding was
as far as when you need to go heavier with pellet weight related
to velocity. the 15.9 jsb's are the only jsb pellet i haven't
tried in my mrod. Idea

can you possibly tell us what energy a kodiak gives
at various ranges? starting at 790 fps (30 fpe). Smile

rsterne · Moderator

According to ChairGun3, the 21 gr. Kodiak has a BC of 0.036 but they don't specify the starting velocity.... I measured the BC at 0.047 starting at 747 fps.... However, I measured the BC at 406 fps as only 0.025.... Both measurements were done over 25 yards.... I'll use the ChairGun BC as it probably represents the best average at your MV and over a 100 yard distance....

MV = 790 fps; ME = 29.2 FPE

25 yards; V = 724 fps; E = 24.6 FPE

50 yards; V = 663 fps; E = 20.6 FPE

75 yards; V = 608 fps; E = 17.3 FPE

100 yards; V = 557 fps; E = 14.6 FPE

The "actual" results would probably be more velocity and energy at 25 yards but pretty accurate by the time you get to 100 yards.... There is about half the energy remaining at 100 yards.... Pretty similar energy retention to the JSB 18 gr. Heavy.... Mr. Green

Bob

sniper · Silver Status Member

fritz · Silver Status Member

How can you give a single, numerical answer for the BC of a given projectile when the BC is an exponential curve (not linear)? Might a more accurate way to do this is average the BC's from muzzle to X yards that the BC is recorded over?

rsterne · Moderator

There is no single answer for the BC.... but it is not exponential or linear.... in fact I have found that it peaks at some velocity or other depending on a lot of factors....

Determining it at the velocity you require experimentally is the only accurate way.... and even then it will vary over the flight of the pellet as the velocity changes....

Bob

Jaxom · Senior Member

Thank you VERY much Bob. Once again you've come throught with precisley the information I was seeking. Now if could only get this bleeping sale done, moved and setup some place where I can put all this information into practice! OY!

FYI, I don't have a scope with mil dot. I had bought a cheap gamo scope for my 1077. That's now on my 2300. And I bought a Hawke scope for my disco. Eventually I'll take the current Hawk I have now off the disco and put that one on the 2300 and get a better Hawke, simlar to the one Bob reviewed, on the disco.

I don't recall anyone but bob ever mentioning these scopes. They're pretty good!

donwalk · Veteran Member

i modded a QB78...i managed to get over 800fps in .22 but got only 3 usable shots per refil... Shocked

back to the drawing board... Embarassed