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6.5 Prometheus (6.5 PRO) 140 gr. 3700 fps

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Unread 11-13-2012, 09:23 AM
Junior Member
Join Date: Mar 2004
Posts: 6
Re: 6.5 Prometheus (6.5 PRO) 140 gr. 3700 fps

Originally Posted by Swamplord View Post
The case water capacity is a typo in the above diagram, it's more like 105 gr. water....
As the owner of AmmoGuide, I can assure you that the 102.9 gr. Case Capacity listed in the AmmoGuide data is NOT a typo.

First, understand that water is extremely dense and it doesn't take much of it to make the 2 grains difference you perceive (about 2% of the total volume in your wildcat's case, actually).

Secondly, 2% (the difference between 105 and 102.9) is an acceptable tolerance for even high-precision instrumentation. In my Navy days as a Nuclear Electronics Technician, the multimeters we used to measure/calibrate voltages of systems directly affecting the reactor core only had to be within 3%.

Thirdly, "case capacity" is a particularly elusive parameter to nail because it is subject to many real-world variables when being measured. Any 2 people trying to measure it are unlikely to come up with the same answer. The temperature of the air & water, altitude, how many times the case has been reloaded, whether the case is clean inside, whether the case has been fired, resized or even HOW it was resized... these and even more factors cause "case capacity" to vary when measured empirically. You may consider "105" to be THE NUMBER but the relevance of your result to the rest of the world is reduced because of this variability.

Plus, since you shoot a wildcat, your cases are certain to have been fired at least once and/or formed in a die before measuring. There is no such thing as a "factory virgin" case in 6.5 PRO - your volume measurement will alwys be affected (slightly) by the specifics of either your rifle's chamber, die size or both.

AmmoGuide calculates the internal volume of the case in grains of water in a uniform manner that assumes virgin, unsized, unfired brass. Therefore, the website's values are much more useful in terms of comparison with other case capacity values on the website because all variables have been removed in the estimates for all cases.

Mike Haas
Owner, AmmoGuide.com
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Unread 11-13-2012, 09:35 AM
Junior Member
Join Date: Mar 2004
Posts: 6
Re: 6.5 Prometheus (6.5 PRO) 140 gr. 3700 fps

BTW, the .347" neck-length in the AmmoGuide diagram isn't a typo, either, despite the fact that your description notes a .342" neck-length.

In the (second set of) dimensions you submitted to AmmoGuide, .347" is what the neck-length works out to (neck-length = case-length - top-of-shoulder). Apparently, either your textual description or submitted case dimensions still aren't exactly correct.

Mike Haas
Owner, AmmoGuide.com
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Unread 11-16-2012, 11:57 AM
Bronze Member
Join Date: Jan 2009
Location: Ketchikan,AK
Posts: 55
Re: 6.5 Prometheus (6.5 PRO) 140 gr. 3700 fps

Swamplord, have you actually fired this chambering? I don't ask you this because I'm disputing your claims but rather to see how user friendly it is overall. Muzzle blast powder selection and fouling. I have been using a 6.5 STW and really love it although I'll be the first to admit it can be a real ******* at times too.

Pressure spikes, weird flyers, and with the real slow powders I get 80 FPS spreads. I assume it is because my 26" barrel is too short???

When I had this rifle built I suggested to the gunsmith that I may like to have my rifle chambered in 6.5 RUM or 6.71 Blackbird and he said no. It is a lighter weight gun so I'm happy but I still wonder what could have been. On the other hand I'm debating whether I should double up for my heavy LR rifle and stay with STW for simplicity sake or go BIG! LOL

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Unread 11-18-2012, 03:10 AM
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Join Date: May 2004
Location: Alaska
Posts: 994
Re: 6.5 Prometheus (6.5 PRO) 140 gr. 3700 fps

Mr. Haas, you are correct... the max overall case length is 2.502" and the base of the neck is 2.155" making the length of the neck .347" as you stated, the reamer print drawn up by Dave Kiff of PTG shows the exact same thing !!

BUT .... the case is trimmed back to my specs of a .342" neck every time the cartridge is fired (if needed) and will be maintained at that length, the finished cartridge will always possess a .342" neck, that is why I repeatedly used the .342" neck length #'s ....

The case capacity is a non issue as you stated, I'm certain the variation in the parent 338 Norma Mag and the 300 Norma Mag will give different case capacities and from different lots of brass and dates of manufacture, always does !!!

I apologize for causing any confusion !

I sincerely thought this wildcat would cause a bit of commotion, guess I was wrong,
Seems that folks are more excited if the improvement in velocity gains is 100 fps or so,
The 6.5 Prometheus might just be too fast and too flat shooting of a 6.5 for the LRH forum members, (is that possible ?) haha
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Unread 04-25-2013, 09:49 AM
Bronze Member
Join Date: Apr 2011
Location: United Kingdom
Posts: 30
Re: 6.5 Prometheus (6.5 PRO) 140 gr. 3700 fps

This cartridge is just what I was looking for! Does anyone have any results from testing that I can sink my teeth into?
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Unread 04-25-2013, 10:09 AM
Silver Member
Join Date: Sep 2012
Location: Northern Colorado, Saint Francis, KS
Posts: 132
Re: 6.5 Prometheus (6.5 PRO) 140 gr. 3700 fps

tagging in
Pic out a spot,a single hair, and aim for that spot.
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Unread 04-25-2013, 10:41 AM
Gold Member
Join Date: Oct 2007
Location: Patagonia Mountains, Arizona
Posts: 770
Re: 6.5 Prometheus (6.5 PRO) 140 gr. 3700 fps

Considering a 264 Win Mag only has 82 grains case capacity and the Prometheus has 102 grains what do you expect for barrel life? Most 264 Win Mag barrels die of throat erosion in under 1000 shots.

That's not to say you shouldn't build the 6.5 Prometheus.

My candle burns at both ends,
It will not last the night.
But ah, my foes, and oh, my friends,
It gives a lovely light.
(Millay 1920)
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