Playing with the Extreme .308

[Fur-n-dirt]

I shot the Extreme for accuracy today...

Filled the gun to 4000psi using a modified Lee115 grain to HP.  My chronograph shows the first shot averages 1035 fps.

I shot three group strings at 4000 psi.  First three shot group was .7" ctc and second group was .5 ctc at 50 yards.  Excited to shoot more and see where the numbers fall.

Not bad considering using 5x scope.

I did notice that the second shot was consistently lower by at least 4"!   The third shot was 1.5 to 2" below that.

After talking to Brent, I'm finding out the gun is designed to shoot heavier bullets and at higher pressures.

I love these bullets, I want to figure out how to get two consecutive shots.  Considering the bullet pressure exerted on the valve, seems a lighter spring can help.   Any suggestions on getting better performance is greatly appreciated. 

More to come.   

[rsterne]

You need to Chrony all the shots to know how to tune the gun.... How can the groups be 0.7" and 0.5" CTC and yet the shots drop 4" and 1.5-2" ?

Bob

[Fur-n-dirt]

You need to Chrony all the shots to know how to tune the gun.... How can the groups be 0.7" and 0.5" CTC and yet the shots drop 4" and 1.5-2" ?

Bob

Hi Bob,

Let me clarify.   When I counted groups, it was based on the "first shot" filled at 4000 psi.   So, the two groups reported was based on shots at 4000 psi.

I chronographed the other day..  At 3800 psi, the velocities for the first three shots were 1054 , 984, and 924.

Today, when I was shooting , the pressure dropped from 4000 psi to 2500 psi after three shots.

[rsterne]

Let me get this straight.... Three shots with 109 gr. bullet (that's what mine weighed, same mold as yours)....

1054 fps = 269 FPE
984 fps = 234 FPE
924 fps = 207 FPE

Total FPE = 710 FPE

Pressure used = 1500 psi (4000 - 2500) = 103 bar.... Do you know what the volume of the air reservoir is?....

IMO, the spring is wayyyyyyyy too stiff for that bullet weight at that fill pressure.... The quickly dropping velocity is part of the reason for your dropping POI....

Bob

[Fur-n-dirt]

Bob,

Yes.  Those numbers stated in your last reply is correct.  That is my situation.

Reality is sinking in that this stock gun likes heavy bullets and at high pressures.  Very high!

It appears that I am way under the bell curve.

Your suggestion about being stiff confirms my initial guess stated at the beginning of this thread.   

I really like this bullet.  I did some calculations and this bullet should fly great with this barrel twist.  And it seems to be the case or else I wouldn't have gotten that .5 ctc group (and I've only just started). 

It would be awesome to have a consistent 3 shot fill with this bullet.

So, the question is, where do I go from here?  Should I plug and chug with lighter springs?   A member had stated using a lighter spring in his extreme, but I thought it was a higher caliber ( need to check).  I guess I could start with this one and adjust?

For tank volume, I don't know; however, OUTSIDE dimensions are approximately 24 inches by 1 3/8"

Thanks for your help guys!

[grumpy]

What hammer spring are you using. My Extreme .308 gets 5-7 consistent shots.

[rsterne]

I would assume the wall thickness is 0.125", which would result in an ID of 1.125" and a safety margin of 3.1:1 at 4500 psi, assuming CrMoly tubing.... That works out to 1 CI of volume per inch of reservoir length.... We need the inside length (front of valve to back of fill fitting), but if we assume it is the 24" you gave, then we can use 24 CI to do the efficiency calculation....

The total volume used for the 3 shots would be 24 x 103 = 2472 CI to develop just 710 FPE, so that works out to 710 / 2472 = 0.29 FPE/CI, which franky, is pitiful.... This is not uncommon with Big Bores, my DAQ .308 Exile ran 0.38 FPE/CI the way I got it, and simply by setting the hammer spring (compressing it fully and then letting it rebound, which shortened it nearly an inch) I got the efficiency up to 0.88 FPE/CI, which while not great, is at least acceptable.... The crazy thing is that I went from 3 shots of 836, 786, 742 from 3000 psi down to 1800 stock, to 6 shots of 826, 833, 832, 824, 811, 792 from 3200 psi down to 2050.... In other words, I lost only 3 fps at 3000 psi, and I got 6 shots instead of 3, and my 6th shot was faster than the 2nd shot stock.... I have no idea why Big Bore manufacturers insist on overspringing their guns, the only logical explanation is that their customers are willing to accept poor efficiency, maybe they get a kick out of hearing that loud BOOM !!! as all the air flies, wasted, out the end of the barrel....

IIRC, fitting a hammer spring preload adjuster is not easy on the Extreme, as the stock is in the way.... I'm not so sure that is the case, perhaps a small screw could be offset near the top of the rear plug, moving a sliding spring seat, but let's assume for now you can't, so you have to change springs.... The easiest thing to do is to get a couple more stock springs and first, try setting one, and if that doesn't shorten it, then cut off a coil or two at a time until you get what you want.... I recently played around with hammer spring preload on my Disco Double, which is unregulated, with the following results....



You will note that with the 50 gr. pellets, the last 4 turns of preload (0.168") did nothing but waste air, and with the 70 gr. bullets, the last 2.5 turns (0.105") didn't gain any velocity either.... However, even though the velocity didn't increase with the preload at maximum, the gun used wayyyyyyy more air.... I'm guessing your gun is currently set up at 4000 psi so that the velocity is way up on the plateau, and the efficiency way down.... One thing for sure, you need to reduce the hammer strike.... and/or run 4500 psi....

Bob

[shamu25]

Fur-n-dirt.....did you get a spare spring from Brent with the Extreme?  They have the stock heavy spring which comes with the gun  (Brent's favorite)  which has the bell curve well above 4000psi and a lighter spring which should get you a bell curve around 4000.  The even lighter spring (Century Spring #2894) which I told you about that I use in my Extreme 45 will drop the bell curve to the lower 3000's and give you more consistent shots as the pressure drops only about 3 or 400 psi per shot.  All of this assumes that Brent is using the same springs in both the 308 and 45.  You could check with him to find this out.  Also as I noted before the velocity only drops about 10% in my gun from the heaviest spring and 4000 fills to the lightest spring and 3000 psi fills.

[Fur-n-dirt]

Grumpy - How the *(&^ are you getting 5 consistent shots?   I have the light spring.  What psi/bullet?

Bob - That's a big eye opener.  I reproduced your calcs on Sike's site.   Definitely room for  improvement!  Thanks again for the numbers, this helps.

Shamu25-  I only got one spring for my gun. :-(  It's the light spring as I've been told.  And I didn't even get the gun case or scope mounts.

I think the consensus should be to go to a lighter spring.  I think I'll go both routes; get the after market spring and request a couple of stock springs from Brent.   Try and work backwards into a consistent shot string.  I just need to be careful ordering springs that don't set over time from material or storage spring position.

Shooting at 4500 psi is not practical.  I have a shoebox that fills up to 4500 or so.  :-(

Thanks!

[rsterne]

Easy solution.... set the spring by compressing it fully before installing it.... That way it will basically never get shorter unless you store it at coil bind....

Bob

[grumpy]

Its not as hard as you think. Check out my buddy Jim's videos on his Extreme .308





He also has a few others with his .308 I have not linked. My gun is vitually identicle to his except I got a slightly longer 30" barrel

Dave

Grumpy - How the *(&^ are you getting 5 consistent shots?   I have the light spring.  What psi/bullet?

Bob - That's a big eye opener.  I reproduced your calcs on Sike's site.   Definitely room for  improvement!  Thanks again for the numbers, this helps.

Shamu25-  I only got one spring for my gun. :-(  It's the light spring as I've been told.  And I didn't even get the gun case or scope mounts.

I think the consensus should be to go to a lighter spring.  I think I'll go both routes; get the after market spring and request a couple of stock springs from Brent.   Try and work backwards into a consistent shot string.  I just need to be careful ordering springs that don't set over time from material or storage spring position.

Shooting at 4500 psi is not practical.  I have a shoebox that fills up to 4500 or so.  :-(

Thanks!

[grumpy]

And I cannot pass up the chance to post a pic of my baby lol

[Fur-n-dirt]

And I cannot pass up the chance to post a pic of my baby lol

Thanks David..   This confirms that you need to be around 4500 psi to be on the bell curve for ~120 grain.   However, he switched to much lighter weight on the fourth shot! (I like Cricket, I bought his left over ammo for the extreme).

For me, its not practical to shoot at 4500 psi, with a 4500 to 4700 psi SCBA tank.

Hopefully, I can come up with something.  The barrel twist should be perfect for this length of bullet.

David, do you have any groups or ammo recommendations that you would like to share?   Greatly appreciated.

Oh, here's mine.    Got a vortex scope on sale.   These extremes are down right sexy.       

[CarsonRatSniper]

Have someone make you a power adjuster, bust out the dremel and take your time fitting it to the stock.
I had to do that with my Corcoran stock - I wanted to be able to tune it so it was a necessity.
I think it came out quite nice!


I would ask the maker of the rifle to manufacture one for you. I think he should offer those as an option anyway.

[Fur-n-dirt]

Have someone make you a power adjuster, bust out the dremel and take your time fitting it to the stock.
I had to do that with my Corcoran stock - I wanted to be able to tune it so it was a necessity.
I think it came out quite nice!


I would ask the maker of the rifle to manufacture one for you. I think he should offer those as an option anyway.

Very nice!   

An adjuster has been brought up to Allen from extreme on this forum.

Well Allen , what did you guys decide? :-)

[Jeremy1982]

Fur,you might want to try shooting this gun and raising the fill pressure by 100 psi each fill till you see the fps drop.I have a 457 extreme and get my bell curve at 4700 psi.Your shoebox will do it.I have a older chain drive model and hits 4700 easily,the burst disc in the back of them are rated to 7000 psi and Brent said the extreme is good till 5000.do you have a inline pressure gauge to see when to cut it off?

[Fur-n-dirt]

Fur,you might want to try shooting this gun and raising the fill pressure by 100 psi each fill till you see the fps drop.I have a 457 extreme and get my bell curve at 4700 psi.Your shoebox will do it.I have a older chain drive model and hits 4700 easily,the burst disc in the back of them are rated to 7000 psi and Brent said the extreme is good till 5000.do you have a inline pressure gauge to see when to cut it off?

Thanks for suggestion, I'm going to try for a bell curve at 4000 if possible.  It's just not practical to shoot at 4500 psi for me.

And yes, I have a nice gauge to measure exact pressure. 

Thanks for the all the feedback guys..  Stay tuned..

[grumpy]

I cannot seem to find my .308 strings but soon as it warms up a bit ill shoot some more. I will say that I use the light spring and only fill to about 3500.  I also plan on tethering the gun soon to get some nice long distance shooting in.

[Jeremy1982]

Tethering it should be lots of fun cause these guns are really accurate.I dont have the set up to tether my .457 but I have filled to 4400 after each shot and gotton 1.5 inch groups at 100 yards with the 245 gr maxi ball's.The Lee REALS aswell

[grumpy]

I'm more then sure sub MOA is possible with mine at 100. I would like to try to push it to 150 next.