+- 6.5 Grendel Forum (https://65grendel.com)
+-- Forum: 6.5 GRENDEL DISCUSSION FORUMS (https://65grendel.com/forumdisplay.php?fid=3)
+--- Forum: 6.5 Grendel ARs and Other Semi-Autos (https://65grendel.com/forumdisplay.php?fid=8)
+--- Thread: 18 vs 20" (/showthread.php?tid=7141)
18 vs 20" - bigbear_98 - 01-24-2014
Ballistic AE. I don't think it changes the BC. It changes the MV to make the numbers right. That is what it did with my 308. The 308 is true true true though. I have been on at 1000 with that rifle on a "watch this" level.
20" 1:9 Krieger w TBAC 30BA hornady sst 123gr
G7 BC, 2436mv (trued @ 750)
BC Bullet Weight Muzzle Velocity Zero Range Sight Height LOSAngle
0.260 G7 123gr 2120@200yrds 100 2.8 in 0°
Altitude Pressure Temp RH WindVelocity Wind Angle
935.0 30.28 Hg. 57.6 °F 22% 6.3 -20
Zero Altitude Zero Pressure Zero Temp Zero RH Max PBR Zero
935.0 30.28 Hg. 57.6 °F 22% 222
Bullet Trajectory
Code:
Range(yards)Drop(in)Drop(moa)Wind.(in)Wind.(moa)Veloc.(fps)Energy (ftlbs)Time(sec)
0 ↓ -2.75 0.00 0.00 0.00 2436 1621 0.00
25 -1.48 -5.66 -0.01 -0.04 2396 1567 0.03
50 -0.59 -1.13 -0.04 -0.08 2355 1515 0.06
75 -0.09 -0.12 -0.09 -0.12 2315 1464 0.09
100 ⊕ -0.00 -0.00 -0.16 -0.16 2276 1414 0.13
125 -0.33 -0.25 -0.26 -0.20 2236 1366 0.16
150 -1.09 -0.69 -0.38 -0.24 2198 1319 0.19
175 -2.30 -1.25 -0.52 -0.28 2159 1273 0.23
186 ↑ -3.04 -1.55 -0.59 -0.30 2143 1252 0.25
200 -3.97 -1.90 -0.68 -0.33 2121 1229 0.26
225 -6.13 -2.60 -0.87 -0.37 2084 1186 0.30
250 -8.79 -3.36 -1.08 -0.41 2047 1144 0.34
275 -11.97 -4.16 -1.32 -0.46 2010 1103 0.37
300 -15.68 -4.99 -1.59 -0.50 1973 1063 0.41
325 -19.95 -5.86 -1.88 -0.55 1937 1025 0.45
350 -24.81 -6.77 -2.20 -0.60 1901 987 0.49
375 -30.26 -7.70 -2.54 -0.65 1866 951 0.53
400 -36.33 -8.67 -2.92 -0.70 1831 915 0.57
425 -43.05 -9.67 -3.33 -0.75 1796 881 0.61
450 -50.45 -10.71 -3.77 -0.80 1761 847 0.65
475 -58.54 -11.77 -4.24 -0.85 1727 814 0.69
500 -67.36 -12.87 -4.74 -0.91 1693 783 0.74
525 -76.94 -14.00 -5.28 -0.96 1659 752 0.78
550 -87.31 -15.16 -5.85 -1.02 1626 722 0.83
575 -98.50 -16.36 -6.46 -1.07 1593 693 0.88
600 -110.54 -17.59 -7.10 -1.13 1560 665 0.92
625 -123.47 -18.86 -7.79 -1.19 1528 637 0.97
650 -137.33 -20.18 -8.51 -1.25 1495 611 1.02
675 -152.16 -21.53 -9.28 -1.31 1464 585 1.07
700 -168.01 -22.92 -10.09 -1.38 1432 560 1.12
725 -184.91 -24.36 -10.94 -1.44 1401 536 1.18
750 -202.92 -25.84 -11.84 -1.51 1370 512 1.23
775 -222.08 -27.36 -12.79 -1.58 1339 490 1.29
800 -242.45 -28.94 -13.78 -1.65 1309 468 1.34
825 -264.09 -30.57 -14.83 -1.72 1280 447 1.40
850 -287.05 -32.25 -15.93 -1.79 1250 427 1.46
875 -311.40 -33.98 -17.08 -1.86 1222 407 1.52
900 -337.19 -35.78 -18.28 -1.94 1193 389 1.58
925 -364.52 -37.63 -19.55 -2.02 1166 371 1.65
950 -393.43 -39.55 -20.87 -2.10 1139 354 1.71
974 -424.02 -41.53 -22.25 -2.18 1115 339 1.78
975 -425.28 -41.61 -22.30 -2.18 1114 339 1.78
1000 -456.35 -43.58 -23.68 -2.26 1094 327 1.85
Range Drop Drop Wind. Wind. Veloc. Energy Time18 vs 20" - cst - 01-24-2014
So..I know due to alot of variables your gonna get different results...but it seems that a bunch of guys are gettin below 2500fps with 20barrels....some get higher..which makes em transonic at long range(At sea level).....that kind concerns me...didnt Bill A. say that you would expect to see 123gr.s go approx 2600fps? That would then make 18" still supersonic at 1000yards.
18 vs 20" - explorecaves - 01-24-2014
cst Wrote:So..I know due to alot of variables your gonna get different results...but it seems that a bunch of guys are gettin below 2500fps with 20barrels....some get higher..which makes em transonic at long range(At sea level).....that kind concerns me...didnt Bill A. say that you would expect to see 123gr.s go approx 2600fps? That would then make 18" still supersonic at 1000yards.
Probably out of a 24" barrel...
18 vs 20" - bigbear_98 - 01-24-2014
LRRPF52 Wrote:What BC does your program kick out for you with the truing feature?
I have been playing with my numbers a little on the app. I entered the muzzle velocity my chrono showed (2460), and changed the bc. I was using Litz's number of .260 G7 for the SST. With the MV of 2460 and a 200yrd fps of 2162 and a 750yrd drop equivelent of the chart that showed to be correct -25.83, the BC drops to .220.
18 vs 20" - LRRPF52 - 01-24-2014
Where did you see a Litz BC for the 123gr SST of .260 G7? I didn't know he did BC analysis on the 123gr SST. I thought he did the 129gr SST at one time, but I don't have his books in front of me.
The 120gr Berger has a G1 of .453 and a G7 of .232, so you would think the 123gr SST would have a higher G7 BC due to the boat tail length and form factor compared to the 120gr Berger.
18 vs 20" - bigbear_98 - 01-24-2014
LRRPF52 Wrote:Where did you see a Litz BC for the 123gr SST of .260 G7? I didn't know he did BC analysis on the 123gr SST. I thought he did the 129gr SST at one time, but I don't have his books in front of me.
You are correct. I am mistaken. Going back to my notes I couldn't find any data from Litz on this and in fact I emailed the maker of the app asking about it and received the same info. I was using the Hornady data and since I have been using the Litz numbers for everything else, I confused myself.
LRRPF52 Wrote:The 120gr Berger has a G1 of .453 and a G7 of .232, so you would think the 123gr SST would have a higher G7 BC due to the boat tail length and form factor compared to the 120gr Berger.
The Hornady number which I was using is .260. When I adjusted the BC to match the true field data, the app spits out .233.
18 vs 20" - SMM123 - 01-24-2014
So if 1100 FPS is the theoretical super/subsonic line of demarcation for given conditions and range, and your round out of a 20" barrel at that line is 1110 FPS (supersonic), or is 1090 FPS (subsonic) from 18" barrel, is the 20 FPS really that big of a difference? Is the target receiving the impact going to give a damn about the difference?
If supersonic kills, then I am perplexed as to why my subsonic 300 AAC Blackout has little problem in dropping game of almost any size at 200 yards while traveling at less than 900 FPS...
Am I missing something? Is it the transition between the two that makes the difference?
18 vs 20" - tackdriver - 01-24-2014
I think the 208grn bullets that people are loading subsonic is the key to that
18 vs 20" - LRRPF52 - 01-24-2014
SMM123 Wrote:So if 1100 FPS is the theoretical super/subsonic line of demarcation for given conditions and range, and your round out of a 20" barrel at that line is 1110 FPS (supersonic), or is 1090 FPS (subsonic) from 18" barrel, is the 20 FPS really that big of a difference? Is the target receiving the impact going to give a damn about the difference?
If supersonic kills, then I am perplexed as to why my subsonic 300 AAC Blackout has little problem in dropping game of almost any size at 200 yards while traveling at less than 900 FPS...
Am I missing something? Is it the transition between the two that makes the difference?
There is still some debate about the transonic range, stability factors, and how to maintain stability factor through it. In aerodynamics, I'm not sure that it is even fully understood. If your transonic range is within close distance to where your max target is, you generally are OK. Trying to hit targets 100yds past that can often be problematic. One theory is that tighter twist will help maintain projectile gyroscopic stability better through the barrier, and Todd Hodnett seems to follow that formula with his rifles, since he likes to push the capabilities of even a 16" .308 AR10 out to 1 mile, as well as his 20" .338 TRG's.
I suspect there is something to this, since my 16" AA Grendel pipe has a tight twist of at least 1/8", if not 1/7.75", and really surprised me as to how well it grouped at 1200yds last summer.
Here are some speed of sound theoretical barriers based on different elevations, and I reference these when running my ballistic data:
Sea level: 1116fps
5000ft: 1097fps
10,000ft: 1077fps
15,000ft: 1057fps
20,000ft: 1037fps
As the air density decreases, the resonance of sound is more diminished. Conversely, the speed of sound in common steel is 20,000fps. The altitudes I commonly shoot in up here in the Rockies are such that I have more distance to work with before passing the transonic barrier.
18 vs 20" - bigbear_98 - 01-24-2014
SMM123 Wrote:So if 1100 FPS is the theoretical super/subsonic line of demarcation for given conditions and range, and your round out of a 20" barrel at that line is 1110 FPS (supersonic), or is 1090 FPS (subsonic) from 18" barrel, is the 20 FPS really that big of a difference? Is the target receiving the impact going to give a damn about the difference?
If supersonic kills, then I am perplexed as to why my subsonic 300 AAC Blackout has little problem in dropping game of almost any size at 200 yards while traveling at less than 900 FPS...
Am I missing something? Is it the transition between the two that makes the difference?
I think the idea of a bullet being supersonic comes from the stability of the bullet and how accurate it can be. From my understanding, the bullet is on it's highest BC until it gets to about 1380fps. Then the BC starts to go down as the bullet goes into the transonic zone. The effects of the atmosphere are greater here too. Then when the bullet becomes subsonic, it changes BC again and has a whole other flight characteristic. If the bullet becomes unstable for some reason in this time, it may not recover before it hits the target.
When a bullet is fired subsonic and stays subsonic, it doesn't experience this. The "killing" factor of the bullet isn't the issue other than some bullets expand better at certain velocities.
This is only my understanding of this topic. I have learned most of this from, believe it or not, shooting 22lr. When shooting subsonic ammo you can get out to 200 yards and still be fairly accurate. When shooting supersonic, the bullet gets real squirrely at ranges of 100 and further.
Again this is only my understanding. Someone will be able to come along and explain it better.
ETA: LRRPF52 came in while I was typing and hit on some of this.
18 vs 20" - LRRPF52 - 01-24-2014
If you study the history of aviation, breaking the sound barrier was a big deal and a major challenge from an engineering standpoint, because the control surfaces of the aircraft behaved differently than in subsonic flight, and stability became a serious issue.
Bullets are spinning thousands of RPM, but stability becomes an issue as well for some reason. One theory is that the Mach wave of pressure built up on the face of the projectile or airfoil changes shape, and can even exhibit resonant disturbances that create strange buffeting patterns that wreak havoc on airfoils, whereas bullets begin to wobble out of their optimum stability spin behavior.
![[Image: sonic-boom.jpg]](http://www.jet-fighter-rides.com/aaa/2010/07/sonic-boom.jpg)
In long-range shooting, we can clearly see the effects of it when an accurate load at 1000yds opens up all over the place at 1400-1600yds. I think there is merit to spinning the bullet faster so that once it begins passage of the transonic barrier, it is more stable than it would be from a looser twist, and Todd Hodnett's work with shorter barrels and tighter twists seems to confirm this hypothesis.
18 vs 20" - VASCAR2 - 01-24-2014
Does anyone know if the rotational speed of the bullet decreases as the velocity decreases. I know a faster twist spins a bullet faster than a slower twist but didn't know if the rotational speed of a bullet would be much different at 300 yards compared to 600 and 900 yards. I know I've thrown in a click or two of windage to account for spin drift but I didn't know if there was a formula for how to account for the spin drift at greater range?
18 vs 20" - LRRPF52 - 01-24-2014
In practical terms, we don't really mess with spin drift because wind is much more of a factor. 1000yd competitors get sighters, and the first shooters in a practical match are the guinea pigs. There are programs that have spin drift data available, but I stopped using spin drift factors years and years ago because the feature is almost useless to me. The reading error for wind is so much larger than spin drift, that spin drift becomes small noise in the solution, and ignored after first round.
18 vs 20" - BjornF16 - 01-24-2014
LRRPF52 Wrote:If you study the history of aviation, breaking the sound barrier was a big deal and a major challenge from an engineering standpoint, because the control surfaces of the aircraft behaved differently than in subsonic flight, and stability became a serious issue.
Semantics...LOL. It wasn't that the control surface was behaving differently, it was the tail control surface (rear tab) was being blanked out by the transonic shock wave. The solution was to move the tail control surfaces up/forward and out of the blanked region (where they behaved normally).
At higher than Mach 1, surface control laws would be reversed...but that wasn't the challenge in "breaking the sound barrier"
18 vs 20" - bigbear_98 - 01-25-2014
So does my understanding have merit or am I in left field?
18 vs 20" - Michael - 01-25-2014
cst Wrote:Can you guys post your average velocities for 18 and 20" barrels with 123 gr. Amax or 123 scenars?
I see such a wide range of velocities and barrel lengths its really throwing me off. I sometimes see 22"+ lengths have velocities you would expect from a 18 so I'm a little confused on whch length to get....I'll be shooting out to 800yards on a regular basis..occasionally 1000.(But i got a bolt gun for that!) thanks!
cst - For Factory A-Max out of an AA 20" (or is it 19.5?), see below
10 Shots
Av 2484
ES 22
SD 6.4
Elevation =2200ASL
Hope this helps.
18 vs 20" - calshipbuilder - 01-25-2014
between 2450 and 2500 depending on the chrono from my 18" LW barrel. 1050 Yards is no problem, had keyhole impacts at 1260. 2500ft, 28.5 inhg
I wanted a 16" for my lilja barrel, but settled for a 20"
18 vs 20" - montana - 01-25-2014
BjornF16 Wrote:Semantics...LOL. It wasn't that the control surface was behaving differently, it was the tail control surface (rear tab) was being blanked out by the transonic shock wave. The solution was to move the tail control surfaces up/forward and out of the blanked region (where they behaved normally).
At higher than Mach 1, surface control laws would be reversed...but that wasn't the challenge in "breaking the sound barrier"
I thought it was the coke bottle fuselage that was the break through for enabling fighter planes to break the sound barrier or maybe I'm going off into left field.
18 vs 20" - BjornF16 - 01-25-2014
montana Wrote:I thought it was the coke bottle fuselage that was the break through for enabling fighter planes to break the sound barrier or maybe I'm going off into left field.
Left field...LOL
The "coke bottle" design (e.g. the T-38 fuselage) makes the aircraft more aerodynamic (i.e. less drag).
One of the reasons the tail of the Bell X-1 was getting "blanked out" was the straight wing design. Notice that today, modern fighter aircraft (and even some of the trash/bomb haulers) have swept wings. That delays the onset of shock wave at the leading edge of the wing (i.e. allows aircraft to travel at a higher speed before onset of the shock wave and the corresponding increase in drag) [shock wave is relative to "normal" of the wing].
18 vs 20" - BjornF16 - 01-25-2014
bigbear_98 Wrote:So does my understanding have merit or am I in left field?
bb_98,
BC is speed dependent as you indicate, but there are changes in BC even at supersonic speeds. Litz's numbers likely includes speeds well beyond the Grendel mv, which might negate his published average BC (i.e. his published BCs are the average of multiple BCs in particular speed ranges). I have his book and modify some BCs based upon mv I have with Grendel ammo (e.g. 129 SST, I eliminated the 3000 fps BC and averaged the 1500, 2000, and 2500 fps BC).
Litz's speed ranges are (fps): 3000, 2500, 2000, 1500
He doesn't bother calculating BC into his average BC at speeds less than 1500 fps since they are not practical speeds (i.e. there is a dramatic drop in BC as the bullet approaches Mach 1 and below).
ETA:
Litz 129 SST G1 = .483 (3000 fps = .520; 2500 fps = .499; 2000 fps = .481; 1500 fps = .430)
I use G1 = .470