KE, Momentum, Speed/FPS, Ethical Kill

Graindrain

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This is not a questions about fobs vs blazers, or Xpedition vs XXXXX. I am only concerned with the numbers.

I am trying to decide whether the current setup I have is worth keeping and always wondering if I have an bow that is capable of ethically killing game or if I should trade for a faster setup.

I have a 2015 Xpedition Perfexion in a 28.5" DL, 60 lbs, D loop, peep, shooting a 400 spine Kinetic Kaos arrow with 100 grain heads with Fobs weighing 431.7 grains, OR with 2" blazers weighting 425.1. The average speed of this setup is 257 with fobs and 259 with blazers.



Doing the math the following is what I find true:

KE= 63.2 foot pounds
Momentum= .492 slugs
FPS= 257
Grains= 431

KE= 63.78 foot pounds
Momentum= .491 slugs
FPS= 259
Grains= 425


Since I have a 28.5 Dl the cams on the perfexion are the smaller ones, and the IBO is 325 ish. I understand this is IBO and not what my set up should be close to but for information purposes I state the facts.

I am not a fanboy and I am not stuck with Xpedition but they manufacture another bow with a shorter ATA and Brace height called the Xcentric 6. Again this is not a just get a brand XXX bow.

The IBO for the Xcentric 6 is 354. The X6 only comes with one type of cam.

Given my set up is approximately 68 FPS lower due to my DL, draw weight, and arrow selection. I present the following scenario and question...

If I were to get the Xcentric 6 and use the same arrows and take off the 68 from the 354 leaving me shooting the same arrow at 286 fps the following is what I calculate...

KE= 78.27 foot pounds
Momentum= .547 slugs
FPS= 286
Grains 431

KE= 78.26 foot pounds
Momentum= .543 slugs
FPS= 288
Grains 425

If the shorter ATA and Brace height lends to a less forgiving bow, is it worth it to switch for the greater KE and momentum?

I don't feel comfortable shooting an animal at 60-70 yards not because of my ability but because of the energy the arrow has at this distance. With the faster bow the numbers don't lie, the arrow has more energy, momentum, more penetration.

Should I trade my setup for a new set up, or is my current setup an ethical rig to kill at these longer distances?
 

PathFinder

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I'd go ahead and do it! I went to a CST ZT this year, and I really don't understand why people say speed bows can't be shot accurately. I've got no problem keeping arrows in the kill zone at 100 yards and even farther. I think you should go shoot the Xcentric, and get it if you like how it feels and shoots. I shot a mature muley buck last year at 55 yards, with a 370 grain arrow going 275 fps. It put the rage hypodermic clean through him. Your current setup isn't bad, but the Xcentric has some serious horsepower. I'd shoot some other speed bows while you're at it too, and shoot your arrows through the chrony while you're at it. They don't all make rated speeds.
 

kodiakfly

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If you're looking for justification to buy a new bow, then yeah go for it. Bigger, faster, better. Those numbers you posted are significant increases. I bought a new bow last year because my bow at the time wasn't shooting as fast and as well as I'd have liked it to. I'd also caution you on planning on numbers for a bow you haven't shot yet. You may not get the full FPS jump you're expecting and then how much did you gain? Not to pee in your Wheaties but I bought a bow, set it up and was let down by 10-15 FPS on what it "should've" been with my set up...not IBO or ATA, but what I figured it'd be.

But for your numbers on lower 48 game, you're fine. I remember back in the 90's IBO or P&Y somewhere stated that they recommend 55 ft/lbs of energy for bull elk at "normal" archery ranges. Back then, I know most of us kept our range at 50 yards. So for brown bear, goat, moose, you may want to look at moving those number around some to optimize them. Even forgetting the numbers, think about the bows we were killing animals with years ago. They're on par with kids bows today as far as speed and energy.

But I think you're fine. I know everyone shoots 90+ yards at animals these days with bows shooting as they do, but I still like to think 50 yards is a good range to plan on and that your set up is more lethal than what a good number of guys are hunting with around the country. Heck, I know a guy that killed a goat last year with a 20 year old PSE with round wheels and aluminum arrows. I doubt he crunched his KE numbers and he got the animal down.
 

tipsntails7

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More animals have died by an arrow going 250fps or less then arrows going 300fps or more. I don't think you need to change anything at all, now if you shoot the new bow, like it more and shoot it just as well then go for it.

One question/ comment, are you sure about your Ibo rating on that perfection? The entire point of the 2 different cams on the perfexion is to optimise the draw lengths. Now if your saying that it's 325 because your 1.5 inches shorter then 30 which Ibo is measured at, then I can understand. But realize the same will be true with the xcentric it's 354 at 70 lbs with a 30" draw. If you deviate from that your "real" ibo will drop as well. So in your case 28.5" draw pulling 60. You lose 15fps for draw and probably another 2-3 for 60lb limbs your looking at 336-337 ish.
 
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Graindrain

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I will get confirmation today, but my understanding is the smaller cams, PX², are not as efficient.

My goals for this bow are to shoot mule deer at extended ranges (50-75), and elk within 40, and try for Nilgai.

I am supposed to hear back soon if the new bow is available. Be on the look out for a 2015 Perfexion for sale in excellent condition.
 

5MilesBack

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I don't feel comfortable shooting an animal at 60-70 yards

Should I trade my setup for a new set up, or is my current setup an ethical rig to kill at these longer distances?

I will start by saying that I love shooting long range, and I'm certainly not a "limits" guy. However, I wouldn't use the above as your justification for a new bow.

Also.......what kind of energy and momentum do traditional guys get at 20 or 30 yards? Don't you at least have that kind of energy and momentum still at 60 or 70? Would you consider all traditional guys to be unethical based on their numbers? If not, then why should you?
 

TJ

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I shoot a Switchback XT, 60lbs @ 28", 432 grn arrows. The only two elk I have shot with this bow went maybe 20-30 yds each. I don't think you'll have a problem.

But, I'd get new bow anyhow!!:)
 
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RallySquirrel

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I keep trying to use this same argument to justify the purchase of a new bow too! I was a little worried about the same thing but everything I have read points to 65#'s KE is plenty to kill an elk at most distances provided you have a clean shot. I have personally witnessed a guy with the 65#KE make a full pass through on a broadside bull at 65 yards. The one I shot last year was only at 30 yards when I shot it and I buried my Montec so far into the off shoulder that I had to use a knife to pry it out. Here is a link to what I believe is a pretty widely accepted KE chart for game:

http://bowsite.com/bowsite/features/practical_bowhunter/penetration/

All that said, I will be making an investment into a new bow to replace my 15 year old Mathews Legacy this year. It keeps killing everything I want to shoot with 65#KE but the new technology has made newer bows an absolute pleasure to shoot.
 

larryschwartz

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You are shooting around 7 grains of arrow weight per pound of draw weight. Since your speeds are plenty fast, as tipsntails mentioned, the only change I would consider is to boost your grains per pound up to around 9 from the current 7. That would give you an arrow of around 540 grains which will do two things for you. One, it will absorb more of the energy being transmitted from the bow to the arrow, which will make it quieter on release. Two, the higher mass arrow will absorb AND KEEP more of the energy as it flies downrange which means it will have higher momentum and greater penetration capability when it gets where it is going. Both are good things and contribute to your goal of an ethical shot.
 
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Graindrain

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If I were to do that my pin gaps would be even bigger making my slider MBG Ascent worthless beyond the 5 pin at 60 yards. Not even sure I would get the 5 pin in the housing.

Using your recommendation:

KE= 58.50 foot pounds
Momentum= 0.529 slugs
FPS= 221 FPS ( This is calculated on the ratio of 3 grains to 1 fps) (540-431= 109 (109/3)=36)) (257-36=221)
Grains 540

So more momentum, less KE, larger pin gaps.

If I don't have a pin for a 60 yard shot, then its pointless to add the weight, but the information you provided is correct. Thank you for the suggestion.
 

jmez

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The calculators are not accurate as you increase arrow weight. They do not factor in increased efficiency of a heavier arrow, they always fall short. The only way to get an accurate number is to actually shoot the heavier arrow and measure velocity. Your KE should not go down with the heavier arrow, it will go up as well if you actually shoot the arrow and get the numbers.

The pin gap thing could be a real issue.
 

Will_m

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The calculators are not accurate as you increase arrow weight. They do not factor in increased efficiency of a heavier arrow, they always fall short. The only way to get an accurate number is to actually shoot the heavier arrow and measure velocity. Your KE should not go down with the heavier arrow, it will go up as well if you actually shoot the arrow and get the numbers.

I would think the pin gap would not be an issue if the calculators are giving the wrong velocity?
 

jmez

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By pin gap being an issue I mean you could run out of room in your pin guard by slowing the bow down.

I shoot 500 gr arrows and on a 7 pin Axcel sight my top pin is all the way at the top and the 7th pin is bottomed out in the housing. If I were to increase my arrow weight much more with that sight I woudn't be able to use the bottom pin for 80 yards anymore.
 
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Graindrain

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I read your post two different ways. I will answer it differently as per the way I understand your statement.

1. There is no question as to the arrow being slower if you add the amount of weight Larryschwartz is talking about. My assumption of the speed could be and very well is wrong but it is an assumption that for every 3 grains you add to an arrow you loose 1 FPS.

Slowing the arrow down that much would cause more of an arch/parabola meaning the pin gap would increase. The sight tape would also be more spread out.

You can only slide your sight housing down so much before the housing interferes with the arrow itself. Right now I cant slide down past 102 yards.

2. If the calculators are wrong then you could be correct in the sense that the sight tapes are incorrect. But given what my sight is right now and the tape that is working out to 80 yards, the gaps would increase so much that I don't think it would fit.
 
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Graindrain

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You beat me to posting a reply lol. Jmez is reading Will_M statement the same way as my response #2.
 

Will_m

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For clarity, I was reading the original post to say the calculators gave the wrong velocity, thus I assumed that meant your bow was likely shooting a heavier arrow faster than what the calculator was giving it credit for. From this, I assumed pin gap would decrease as velocity increased, thereby doing away with a pin gap issue, or at least lessening it as compared to what the calculator indicated.
 
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