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The wide and wonderful range of a short-stroked D-54

2/21/2015

10 Comments

 
Some days ago a friend asked:  "¿why 'de-tune' a large airgun to obtain the performance of a smaller one?"

We were talking then about the Diana 280. A small, handy carbine that was designed from the ground up to yield 12 ft-lbs. Sort of a "short action" D-34.

I obviously understood what he was referring to by "de-tune", but then it struck me that the specific way of putting it was very American.
If you "tune" something it is to get more power out of it, ¿Isn't it?, ¿how could there possibly be ANY other understanding of the verb "to tune". At least within the airgunners' circles.

That exchange prompted a train of thought that carried from my insistence in "tuning for optimal stability", to the sometimes jokingly expressed recriminations about issuing D-54's at "NERF gun level" of 12 ft-lbs.

Since there is precious little I can do for the time being between the weather and my health, I decided to dig on my records, do a little bit of experimenting (not much, mind you), and then respond with some facts.

Usually, when I build a CCA WFTF D-54 "engine" (understanding this as the metal powerplant heart of the house section of the gun), I have to cut off 2 coils and change from full power springs. 
¿Why? 
To answer that we need to go back to how the Int'l HMO Piston was designed. 
Back in 2007 I was designing the piston to yield at least 12 ft-lbs when used in Latin America. 
Latin America has a number of country capitals that are above the 7,500 feet above sea level range.
Piston airguns do not have their own air supply, they work with whatever is available in the environment.
Experience had shown that at that altitude, power output was reduced to about 84% of what could be obtained between sea level and the first 1,000 FASL; so the piston was designed in such a way as to yield 14 ft-lbs. at sea level. 
Some calculations and a little final tweaking yielded the current spec for the Int'l HMO piston.


So, what CAN you get when you put an Int'l HMO piston in a D54?

I gathered some data, ran some experiments, and these are the results:
Picture
As you can see, it is fairly easy to reach the maximum velocity/stability region of normal skirted pellets, either with the springs "as is" or with about ½ the maximum spacing available.
Cocking effort of all these setups was under 23# of peak force.

It is important to mention that even with 13 mm's spacing up, the spring is much less stressed than the OEM arrangement where the stroke of the OEM piston requires a compression of a full 100 mm's. So expected life of the spring should be quite interesting. Perhaps not the 20-30,000 rounds of the least stressed WFTF version, but interesting none the less.

¿Could you get more power?
A little, yes, but the spring is fast approaching the region of diminishing returns.

In this context, if someone absolutely WANTS to shoot heavy pellets, the full stroke gun is the only option. 
I emphasize the word WANT because when you "tune" for stability, the BC of the 8.44 pellet approaches real fast the BC of the 10.3. Thereby negating the perceived advantage of the heavy pellet either in trajectory or in wind deflection.

In most instances I've seen in the field, driving hard the 10.3 pellets usually yields BC's that are inferior to the lower weighted projectiles driven to reasonable speeds. And by driving hard I mean that even a very capable engine like the D54 is not really made to yield more than 17½-18 ft-lbs in 0.177" cal. which means a 10.3 grs. pellet having a MV of 875 fps. Not as an airgun. Guns that yield more than that are usually dieseling to different degrees and that, again, usually carries over an increased probability of pellet deformation upon firing, which will degrade the BC of the pellet in flight; and inconsistencies as fuel is burned up.

In larger calibers, up to 0.22", the engines are quite capable of delivering more power due to an increase in Expansion Ratio.
For anyone looking for a nice, steady, easy to use "Hunter" rifle, I would think it hard to find something that can cock as easily, return as much energy per lb of cocking force as these setups do, and be as consistent as the good old D54.

Sorry I cannot post targets; with so much snow I found it impossible to do any testing.

Do NOT put too much emphasis on the uniformity of MV's. As long as MV's are within an extreme spread of 15-17 fps, which SHOULD translate into Standard Deviations of 5-6 fps. you should not worry too much. I've seen too many rifles shoot very uniform MV's just to be disappointed at the poor results where it counts: AT the TARGET.

Keep well and shoot straight!
10 Comments
Umair Bhaur link
2/21/2015 15:53:33

Great article. Better understanding of piston springers.

Thanks

Reply
Mike Hagerty
2/25/2015 04:12:57

I enjoyed the article.
I don't understand it all but that is because I don't understand the D54
as well as I should.
I own one and can not find any detailed information on how to set up
mainly the adjustment of the recoil system.

Thanks again for the article.
Mike Hagerty
D54 22cal

Reply
Hector Medina
2/25/2015 09:47:56

Mike;

Tuning the D54 sled system for each specific power level is relatively easy:

Turn in the rear sled screw, the one that regulates the pressure put into the ball that locks the action in its cocked position, in as much as it will (GENTLY), go.
At this point the gun should lock up and not slide rearwards even upon firing.

Now, turn out the screw by complete turns till the sled releases the action upon firing. Once you find that point, then turn in the screw one full turn. It should lock again.
Now turn the screw out ½ turn, if it releases, turn the screw in ¼ turn. If it locks, turn the screw out 1/8 turn and test. Repeat with finer and finer adjustments till you get to the point where the sled JUST releases.
Once you find it, turn the screw out ¼ turn and loc-tite in place.

HTH

Reply
Michael Hagerty
2/25/2015 14:51:06

Hector
Thanks for the information.
I am just starting with air guns and it seems that springun tuning information is harder to find than it is for compressed air guns.

thanks
Mike

Reply
Steely Dan
3/3/2015 19:07:36

Hi Hector

Sorry to go OT but I have a ' newbie ' doubt about zeroing the gun, say at 10 yards.
The doubt is this -
From where do you measure the distance to the target ?
There seem to be 4 options -
1) The end of the barrel.
2) The pellet position.
3) The Objective Lens.
4) The Ocular Lens.

How do you zero your rifle ?

Thanks
SD

Reply
Hector Medina
3/4/2015 02:57:26

Hello Dan!

What if I say "Neither, either, or all?" LOL!

Zero Distance is not as important as it is made to be. You may need different zeroes for different shooting styles / games.

In FT and in other precision shooting disciplines, the distance difference between the lenses in the scope, the pellet position, or the muzzle is almost irrelevant.

What IS IMPORTANT, is that you do it ALWAYS the same.

PERSONALLY, and I stress it because it is a very personal choice, I measure from the scope's turrets when the rifle is scoped. From the muzzle when I use peep sights and tunnels.

BALLISTICALLY speaking, range should be taken from the muzzle because before the pellet leaves the bore, no other action except propulsion can have any effect on the pellet's trajectory. The pellet inside the bore has ONE degree of freedom. It either goes forwards or not at all.
Once it leaves the barrel is when gravity, wind, and other factors can start affecting the trajectory.

OPTICALLY, I want to do all measurements from the turrets because that is pretty close to where the erector and parallax correction lenses are, and in most instances is pretty close also to where the reticle is.
For the same reason I use the front sights/muzzle to measure distances when operating with iron sights.

For MATCH purposes, distances are measured from some pre-defined point, like the table you must not touch with your body in Olympic Air Rifle and Air Pistol, or the shooting line in FT.

The last thing I need to say is that unless you are dealing with a special purpose airgun, a zero of less than 20 yards/meters is quite an exercise in frustration. Almost anything under that distance will be a first zero and you will have a second zero much further down the road.
For example in my 12 ft-lbs guns, if I zeroed at 10 yards, I would be dead on at 50 again, that would make me hold UNDER for all shots except the two extreme ranges, I do not have too many hash marks above the main crosshair in my scopes and so I do not do it like this. BUT IF I DID, then it would make much more sense to zero in at 50-55 yds. You have more control of what is happening and small distances like the foot or so between lenses, chamber and muzzle do not matter any more when you compare them to the longer ranges.

JMHO

Reply
Steely Dan
3/6/2015 19:19:22

Thank You Hector, for the detailed explanation.
I remember reading a recommendation in the Yellow that zeroing at the apex of the pellet trajectory has some advantages. Let me try it out and see if it makes things easier.
I'd also like to state my appreciation for your extremely informative articles. Most invariably require re-reading but I'm sure that all your readers value your insight and the willingness to share.

Thanks
SD

Reply
Hector Medina
3/7/2015 04:50:37

Hello Dan!

Thanks for your kind words.

I try to put as much information as possible into short paragraphs. At least that is what I have been told to do by the "pro's" in the writing game.
It seems that the current culture is for short "sound bytes". People loose attention and concentration on anything that is long. BUT, on the other hand, I do not know how to write half-truths. Believe me I take some time re-reading and re-writing everything to make it as concise, yet true, and complete, as I can.

Sorry that this means that sometimes, some paragraphs deserve a "double take".

I'll try to do better.

Thanks again!

Un abrazo!




HM

Reply
Nathan Hubbard
1/20/2016 19:38:07

Question regarding the sled adjustment setting on the M54. Sometimes, when I point my M54 upward at a fairly steep angle, the action will "fall back" to the fired position even though I have not fired it. Would that be a sign that the tension on the system is too loose? Should the action remain "in battery" regardless of the angle?
You did not mention any adjustments or tightening procedures for the front sled screw? What should be done with that?
Thank you
V/R
Nathan

Reply
HECTOR J MEDINA GOMEZ
1/21/2016 10:56:10

Hello Nathan!

Yes. If the rifle goes "out of battery" just by pointing it up, it is DEFINITELY too loose.

There are several methods of adjusting the sled, the factory has one, each user has his, I am telling you what is MINE. Derived over more than 10 years of gunsmithing this particular model:

The front end CANNOT be adjusted in factory models (it can in CCA WFTF models). SO, keep the front stock screw tight and loosen the rear stock screw.
UNDER the rear stock screw, there is a grub screw. This is the screw that adjusts the sled system.
Tighten the rear sled unit all the way in, but do NOT force it, just reach the end.
Then back out two turns.
Now, in a HORIZONTAL position, cock and load, and shoot into a safe backstop holding the gun tightly with your forward hand, loosely with your trigger hand.
If the sled releases, then tighten 1/2 turn and repeat.
Repeat till it stops releasing.
Now back 1/2 turn and come in 1/4 turn. If it still releases, then come in another 1/4 turn. It SHOULD stop releasing.
Now back 1/8 of a turn and if it releases, then back off another 1/8 of a turn. If it does not, then back off 1/4 turn.
Loc-Tite this adjustment.
Now install the rear stock screw and check that everything works well.
IF IT DOESN'T, then your stock may be warped and you will need to "align and balance" the action before it works perfectly.

Stock screws in D54's do NOT have to be dead tight, as there is an elastomer washer between stock and action. So, do not overtighten the stock screws.

If you follow this procedure, you will see that no matter where you point your gun, the action does not come "out of battery".

CCA D54's get two adjustable buttons on the front sled installed. This allows not only to align and balance the action, but it also takes up the natural wear of the very heavy usage needed for optimal performance under competition stress.

HTH


Hector

Reply



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    Hector Medina

    2012 US National WFTF Spring Piston Champion
    2012 WFTF Spring Piston Grand Prix Winner
    2013 World's WFTF Spring Piston 7th place
    2014 Texas State WFTF Piston Champion
    2014 World's WFTF Spring Piston 5th place.
    2015 Maine State Champion WFTF Piston
    2015 Massachusetts State Champion WFTF Piston
    2015 New York State Champion WFTF Piston
    2015 US National WFTF Piston 2nd Place
    2016 Canadian WFTF Piston Champion
    2016 Pyramyd Air Cup WFTF Piston 1st Place
    2017 US Nationals Open Piston 3rd Place
    2018 WFTC's Member of Team USA Champion Springers
    2018 WFTC's 4th place Veteran Springer
    2020 Puerto Rico GP Piston First Place
    2020 NC State Championships 1st Place Piston
    2022 Maryland State Champion WFTF 
    2022 WFTC's Italy Member of TEAM USA 2nd place Springers
    2022 WFTC's Italy
    2nd Place Veteran Springers

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