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Post by mh57 on Aug 25, 2018 5:35:15 GMT -8
Does anyone now how to find the drag effect and ballistic coefficient for a crossbow arrow? I want to use the ballistic calculator on both my ATN XSight II and my Thor4 with my crossbow but I can't seem to find this information any where.
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mexicanjoe
Top Contributor
Posts: 208
ATN Products Owned: Smart HD Optics
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Post by mexicanjoe on Sept 4, 2018 18:59:45 GMT -8
Wow. Got me on that one. I am a rifle guy. My BC works great after this past weekends tests.
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odin
Expert Contributor
Laxing out in the land of Rugby, Hunting and Fishing.
Posts: 726
ATN Products Owned: Smart HD Optics
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Post by odin on Sept 4, 2018 19:47:45 GMT -8
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odin
Expert Contributor
Laxing out in the land of Rugby, Hunting and Fishing.
Posts: 726
ATN Products Owned: Smart HD Optics
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Post by odin on Sept 5, 2018 0:30:50 GMT -8
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Widowmaker
Frequent Contributor
Posts: 85
ATN Products Owned: Smart HD Optics
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Post by Widowmaker on Feb 27, 2019 11:36:19 GMT -8
With the ATN X-Sight II HD 5~20 on my Ravin R-20, I use the G1 drag with a BC of 0.08. Height is set to 3.93". I use a reversed 20MOA picatinny adapter to move the scope farther to the rear and raise the objective lens in the front more in line with the crossbow frame rail. (Might not be needed, do your own experimenting) I use 400gr Ravin .001" run out match grade arrows. Both for target and hunting. My arrow speed was originally set to 430 FPS per mfg specs.
Thru some experimenting, I believe the trick is not so much the BC, (I have used up to 1.2 without much POI change at these slower speeds) but actually getting precise arrow speed for the ballistic calculator to function correctly. What I had to do was enter all the ballistics info into the current profile and dead-nuts zero the scope at 30 yards. 20yds would be better, but without the blue tooth range finder, you can't set the yardage on the X-Sight II 5~20 below 27 yards.
Anyway, spend as much time as necessary to absolutely zero the scope at 30 yards. Then set a target at 60 yards. Also get this distance as accurate as possible for better math calculations. Manually enter your range to 60 yards. With any distance shot, be extremely mindful of scope Cant. Make sure it's as level as possible. Take a shot.
If the arrow POI is below your POA, then reduce the arrow speed in the settings by 5 or 10 FPS. (That lowers the reticle in the scope, meaning you have to raise the crossbow back to POA) If it hits high, increase your FPS in your profile settings until the arrow drops into the slot. This should set your correct arrow speed and make the ballistics calculator work for you. Mine hits dead nuts from 30 out to 135 yards. If I hold under an inch or so with it set to 27yds, a close-in shot is hunt worthy. This speed adjustment trick probably works the same for a rifle. By adjusting the speed to actually match the bullet drop and range vs. the bullet mfg generic ballistics fps information may make your rifle more accurate as well. Chrono is the best place to start.
Shoot Straight.
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Post by kaperro on Aug 15, 2019 8:11:18 GMT -8
The coefficient of drag relies mostly on the cross-sectional area perpendicular to the direction of flight and the speed of the projectile. The length of the projectile will also have some to do with it. Assuming that an arrow and bolt are identical in weight, cross sectional area and are launched at the same speed (assuming there are not massive differences in length) they will have very similar coefficients of drag and decelerate and very similar rates.
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Post by cat on May 29, 2020 2:58:41 GMT -8
With the ATN X-Sight II HD 5~20 on my Ravin R-20, I use the G1 drag with a BC of 0.08. Height is set to 3.93". I use a reversed 20MOA picatinny adapter to move the scope farther to the rear and raise the objective lens in the front more in line with the crossbow frame rail. (Might not be needed, do your own experimenting) I use 400gr Ravin .001" run out match grade arrows. Both for target and hunting. My arrow speed was originally set to 430 FPS per mfg specs. Thru some experimenting, I believe the trick is not so much the BC, (I have used up to 1.2 without much POI change at these slower speeds) but actually getting precise arrow speed for the ballistic calculator to function correctly. What I had to do was enter all the ballistics info into the current profile and dead-nuts zero the scope at 30 yards. 20yds would be better, but without the blue tooth range finder, you can't set the yardage on the X-Sight II 5~20 below 27 yards. Anyway, spend as much time as necessary to absolutely zero the scope at 30 yards. Then set a target at 60 yards. Also get this distance as accurate as possible for better math calculations. Manually enter your range to 60 yards. With any distance shot, be extremely mindful of scope Cant. Make sure it's as level as possible. Take a shot. If the arrow POI is below your POA, then reduce the arrow speed in the settings by 5 or 10 FPS. (That lowers the reticle in the scope, meaning you have to raise the crossbow back to POA) If it hits high, increase your FPS in your profile settings until the arrow drops into the slot. This should set your correct arrow speed and make the ballistics calculator work for you. Mine hits dead nuts from 30 out to 135 yards. If I hold under an inch or so with it set to 27yds, a close-in shot is hunt worthy. This speed adjustment trick probably works the same for a rifle. By adjusting the speed to actually match the bullet drop and range vs. the bullet mfg generic ballistics fps information may make your rifle more accurate as well. Chrono is the best place to start. A quicker way to reach your goal is to move from 30 to 60 without adjusting your scope (aim high enough to still hit the target!). Now measure the drop of the arrow with a tape measure. Hold your Bow exactly at the same up/down angle as your test-shot!! Now adjust the distance to 60 and change your BC value until the drop (up arrow) displayed on the screen (4K Pro shows this in left bottom corner) is the same that you have measured before. The BC should be somewhere between 0.05 and 0.13. For my Ravin R26 I ended with this data: zero-distance 40 m 38 cm drop at 60 m Sight Height 88.865 mm (Arrow diameter * 0.5 + Scope Diameter * 0.5 + dist. scope-arrow) (The sight height increases towards the back of the scope! Don't know where to measure. Probably at the CCD?) Bullet Weight 403 gn v0 = 122 m/s (= 411, 400, 400, 396 fps crono measures) BC = 0.118 Use the App to change the BC and you can watch the drop change whenever you hit apply. You can use that also for firearms if you do not know the BC, but you know v0.
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Post by fasyankee on Aug 28, 2021 9:17:16 GMT -8
Sorry to drag up such an old post but I just mounted my 5-20 4K pro ABL1000 on my center point amped 415. I slowed the fps by using longer (22”) and heavier bolts to avg 357FPS. Sighted in at 30yds and it’s consistent there. Moved the bag out to about 55yds and I’m out of adjustment space. The range in the top left corner just says INF and the adj calculations on the bottom left say “null”. I’m assuming the a 20 MOA rail will fix the problem? There’s a UTG one on Amazon for @$20, I’ve had good luck with their rails/rings before.
Another thing I’m finding is that my pocket rangefinder said the bag was at 30 yds but the ABL ranged it at 25 yds, anyone have an issue like that?
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