What's considered an acceptable Extreme Spread?

Depends on purpose, accuracy required, range and mv.
ES of 10 fps at 800 fps is a good result to aim for for 25 shots (0.0125 x velocity.
That is say 11fps or 12 fps at 900; 9 at 700; 8 at 600; 7 at 500.
But the top rifles and pellets at the recent USA BR Championships would have been probably doing around ES of 5 fps for a card of 25 shots plus sighters.
I know the Thomas rifles Mike Niksch makes do that and the results are there eg: Top 10 Open Class:

US Open Class
1 Marvin Reynolds 746 Thomas
2 Matt Kellerman 745 USFT
3 Ron Silveria 743 TM-1000
4 Mike Niksch 742 Thomas
5 Doug Miller 740 Thomas
6 Roger Lovett 738 USFT
7 Dick Strever 736 Thomas
8 Linzie Boise 736 Thomas
9 Vipha Miller 735 Thomas
10 Pete Wass 735 Thomas

Sub 20 fpe HV Class:
Heavy Varmint Class
1 Ron Silveria 747 TM-1000
2 Mike Niksch 741 Thomas
3 Doug Miller 739 Thomas
4 Linzie Boise 735 Thomas
5 Paul Bendix 734 Thomas
6 Matt Lababedy 734 TM-1000
7 Marvin Reynolds 733 Thomas
8 Tony Starr 733 HW-100
9 Vipha Miller 732 Thomas
10 Steve Ware 732 TM-1000

Hunting accuracy to 50 yards probably does not require better than 20 fps ES but it is a poor setup that won't give ES of 10 to 15 fps for 10 hunting shots..

Inch ctc 100 yards 5 shot groups require about no more than 8 fps spread assuming the gun and shooter are at the top of the game and conditions are excellent for the process. The vast majority of the possibly 100 sub inch groups I have shot at 100 yards would have had an ES of 5 fps or less for the five shots at around 880 fps mv.

Kind regards, Harry

Realistically, I'd say 30fps. I believe I saw this listed somewhere recently too as a bit of a standard- for pcp shot strings.

Here was a typical shot string from my unregulated 28 fpe RWS Excalibre. It is year 2000 technology from FX, sold in USA as the Tarantula, elsewhere as the FX 2000 and other names. I have had it since 2002 and it had shot over 70 sub inch 100 yards ctc groups by 2007.
As you see from a reservoir of just 203 cc it shot most of these 31 shots with under a 10 fps extreme spread shot down from 195 bar to 140 bar.



I would presume that later unregulated rifles, developed more recently, should do as well or better especially as reservoirs are often much bigger now.
Regulated rifles should definitely do as well.
My regulated BSA Hornet hunting .25 cal rifle has done 25 shots with an ES of under 7 fps from a 165 cc reservoir filled to 250 bar.

Mean 884.57 fps , 44+ fpe.
Range 881.4 to 887.6 fps
Extreme spread 6.2 fps for 25 shots
SD 1.50 fps

Pellet .25 cal JSB King Exact Pellet, weight 25.4 gr

These are real figures to aim for but as others have written 20 to 30 fps ES is acceptable to many shooters, particularly if they shoot mostly at close < 50 yd range. ... Kind regards, Harry.

Thanks Kris. I presume you refer to my graph above.
That is the practical velocity spread for that rifle for my purposes - and still is.
The maximum fill pressure of the rifle's 203 cc reservoir is 200 bar. 195 bar to 140 bar gave that string at 28 fpe which was very adequate for my purposes with 4 eight shot magazines.

I would presume most other shooters / hunters also settle on the optimum sweet area of their rifles' shot strings for their purposes and act accordingly.. No practical purpose is served by running a rifle down until it dumps the remaining air; or in shooting it in the field, or in wasting pellets over a chronograph until the pellets literally dribble out the muzzle.
New shooters and old ones too generally want to know what is the useful capacity of their rifles. The above graphs show mine; one unregulated at 28 fpe; one regulated to 44 fpe.

Now, what was the problem you have with folks publishing the practical and useful flat sections of their unregulated rifles' power curves thus showing the true useful numbers? You might correct me if I misread the tenure of the "love" and "come on" bits ........ Kind regards, Harry.

What gun do you have?

I am not qualified to help you with those particular guns, but I bet azuaro and others are!! Sure he or others will reply soon!!

All PCP's can be tuned. I am not sure what adjustments are available on each and every gun, but in general you are trying to balance the power plant per your intended outcome. I look for the most power that will still give me great accuracy. This means that my shot count will be much lower, but the goal of a low ES is still the same. I change hammer spring weights and lengths as well as adjust the preload of the hammer spring to help in achieving my tuning goals. The hammer striker is adjustable on the marauders that I work on as well. This basically changes the stroke length or the hammer travel which is another useful tool in the tuning process. You can go even deeper and modify or alter the valve and ports in an attempt to reach your goals. In guns like yours that are regulated, the reg can also be adjusted for different pressures. There are many variables in tuning, but you are simply trying to make the hammer strike the valve with enough energy to open the valve far enough and long enough to reach a certain velocity without wasting air. You are also striving to make each hammer strike as consistant as possible. There are many other variables that have an effect on the outcome. I am just a hobby tuner and hope that my simple explaination might help.

...

I have been lucky to get a 12fpe AA S400, that has a spread of 10 fps from the first shot to shot 80, with the 'sweet spot' between shot 30 and 65 having a spread of 3 fps. Not many shots in the sweet spot, but more than I need for a 25 target BR shoot.

I'm intrigued by the 300 shot capacity of the regulated FX Wildcat and would be interested if an owner can post a shot string of fps.

 For field target shooting from 10 to 55 yards 20fps ES is a long time standard MAXIMUM spread. Less would naturally be better.


John

"kris"not long time ago I did a calculation of change of POI when the v0 changed by 10 or 30fps,
these here are for a sub12 gun, so for you guys it is a worst case scenario, your numbers will be half or third
http://imgur.com/G9CEuqi

Click to expand...

Hello Kris,

I don't know the equations to model it, but your calculations have a few assumptions that introduces error, the most significant being the pellet velocity decays due to the Ballistic Coefficient.

As I say, I don't know the correct equations so I turned to Chairgun. I put the values of 600 and 570 fps (approximately the same as the m/s value you used) with a 10.34gr pellet (i.e. sub 12 fpe) into Chaigun, the POI at 100 yds for each velocity, indicate a difference in poi of approximately 17cm (7 inches).

My interest was piqued to understand how hard we 12 fpe shooters have it, so I put in 900 and 930 fps with a 21gr pellet into Chairgun. The difference in POI at 100 yds is about 5cm (2 inches).