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Author Topic: RDB Accuracy Thread  (Read 2446 times)
HBeretta
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« Reply #20 on: May 18, 2017, 06:46:23 PM »

HB' do you have any heavy bulletted loads? I don't remember the twist on the RDB (1:7 or 1:Cool, but I remember thinking to myself that it favors heavier bullets. Mine certainly appears to reflect that.

not currently but have put 69 & 77gr through.  i predominantly shoot indoors, but the best i've done with my RDB was with black hills 77gr OTM shooting outdoors.  well i should say 77gr only once on the mentioned outdoor occasion.  that stuff is too expensive at just over a $1 per round.  i was hitting under 2" groups and several 1" or a tad above.  i didn't see noticeable results with 69gr, but then again i'm not as consistent as i'd like to be; depends on how much coffee i've had that morning  Cool.  outdoors for me is much better as everything is bright and clear.  problem is my indoor range is so much closer.  

kel-tec website has it listed as 1:7 twist although not indicated on the barrel.  i believe the pre-production guns were 1:9.

i'm going to push for more outdoor shooting with 55gr and push a little more for accurate groups.  i tend to be lazy and casual about it even when curiosity has me going through the motions.    
« Last Edit: May 18, 2017, 06:48:22 PM by HBeretta » Logged
Potss
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« Reply #21 on: May 18, 2017, 11:39:46 PM »

Just so folks are away, twist has basically no effect on accuracy.  The bullet either stabilizes or it doesn't.

And if you are shooting groups at 100y, it is actually better to use high quality lighter flat based bullets all else being equal.  They are easier to QC so they will generally form a more even seal when leaving the barrel unlike the heavier rounds which all have boat tails to boost BC.  Inside 100y, the low BC of flat base match bullets doesn't matter, it is too short a range.  That is why you see Berger 55gr match bullets with a flat base, and not coincidentally they are the most accurate load I've found yet...at 100y.  Push it to 300y and the TMK will reign supreme (until I get the 70gr RDFs shooting).  But if you are testing for mechanical accuracy of a gun at 100y the best rounds to use are stuff like the Lapua 55gr, Prime Match+ 55gr, and HSM Berger match 55gr.  Although I will say in this case the difference between flat base and non-flat base is probably outdone by simply finding factory ammo that best fits your rifle's nodes if you aren't a handloader.  FGMM is certainly worth trying for that alone.
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BrianK
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« Reply #22 on: May 19, 2017, 01:02:48 AM »

Potss your assertion flies in the face of the history of bullet stabilization garnered by many thousands of people and tests over decades. An over stabilized bullet or understabilized bullet might not be stabilized, or it could be. That's basically what you wrote (That's an assumption I make since you used the word "away" and not "aware").  A statement of that sort really nails it down! (If it's stabilized it's stabilized. [basically])

The fact is that longer bullets require faster twist rates to get the spin they require to stabilize, and shorter bullets generally respond better to slower twist rates. If you don't understand that you need to go back to Ballistics 101. Any experimenter can prove that to themselves by playing with longer bullets stabilized at high twist rates and at high velocities and then lowering the velocity (spin rate) and watch the keyholes on the paper. It's pretty basic stuff. It's the reason the industry went from 1:10 or 1:12 for 55 grain bullets for normal velocity .223 or 5.56mm to 1:7 twist on the M-16 when they changed bullet weight to the SS-109 load with the 62 grain bullet. This is kindergarten basic ballistic knowledge. Yeah, I know knowledge has been lost today, we now have the internet and BS knowledge. So very much is being lost merely by folks not proving things for themselves and repeating garbage knowledge. By repeating BS many times it doesn't make it true.
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patrick711
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« Reply #23 on: May 19, 2017, 07:10:00 AM »

Potss, I thought you were being sarcastic at first. OMG
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HBeretta
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« Reply #24 on: May 19, 2017, 12:51:46 PM »

It's pretty basic stuff. It's the reason the industry went from 1:10 or 1:12 for 55 grain bullets for normal velocity .223 or 5.56mm to 1:7 twist on the M-16 when they changed bullet weight to the SS-109 load with the 62 grain bullet.

potss...lol...you lost me man.  going into my RDB purchase there was debate on whether or not the production guns were 1:7 or 1:9 twist as it's not indicated on the barrel.  i hadn't read anywhere either of someone figuring this out for themselves.  even going into my rdb purchase with the notion of getting either twist i was contemplating bullet weights if i was going to shoot for accuracy.  i can't speak for other RDB owners out there but i went the old school route in figuring out my twist rate with a cleaning rod, painters tape, sharpie and tape measure.  it appears what kel-tec has listed on their website is accurate - my rdb has a 1:7 twist rate. then again, with the subtle changes such as the new bolt and so on, that isn't announced by kel-tec, i wouldn't be surprised if the early production RDBs were 1:9 twist; up to each owner to measure their twist rate.

my worst groups and outings have been with 55gr.  with 69gr and up, with 77gr being the best i've shot, i've never walked away going damn i really sucked today, but have on several occasions with 55gr.  the RDB has a 17.3" barrel, so i had the notion going into my purchase with the assumption i'd be getting a 1:7 twist that i'd be shooting more accurately with heavier bullets rather than stretching it with a 1:9 twist.  basically i figured the RDB wouldn't shoot 55gr worth a damn which really hasn't been the case.  i've shot pretty damn good groups, but with greater deviation than the heavier loads like 69-77gr as expected though.

with regard to twist rates, i figured it was common knowledge the progression to faster twist rates to help stabilize along with evolving to the heavier bullet trends.  this and the military adopting for 62gr back in the 80s along with the purpose to stabilize the longer tracer rounds.  that and the 1:7 being ideal for 10.5" SBRs with regard to 55-77gr loads that a slower twist rate wouldn't be able to handle.  well basically getting away from the slower twist rates period, that won't stabilize the heavier loads - like the 1:14/1:12 twists.  

    
« Last Edit: May 19, 2017, 12:54:41 PM by HBeretta » Logged
Potss
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« Reply #25 on: May 19, 2017, 03:44:08 PM »

Ok, a lot of misunderstands to clear up here.

1.) @ BrianK, you don't seem to have understood my post.  A bullet that is under or over stabilized by definition isn't stabilized.  Perhaps if I restate my initial statement this way it will make more sense to you: once a bullet is stabilized properly, twist rate does not effect accuracy.  In other words, a 1:7 will stabilize a 55gr, as will a 1:9, and all else being equal they will exhibit the exact same level of accuracy since the round is stabilized. Do you understand what I'm saying now?  Of course bullet stabilization depends upon twist rate and bullet characteristics (although there is a little more to it than that, JBM does a good job here: http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi), and if you'd read my prior posts here you'd know that I have made very specific posts on this subject. Perhaps instead of going on a tirade about insanely obvious s***, you should have stopped making assumptions and asked what I meant (which I thought was blindingly obvious).  Also it should have been obvious by the second part of my post what I was referring to.

2.) @ HBeretta, you realize you quoted BrianK and not me right?  And you realize BrianK is talking about the M16-->M16A2 progression, not the RDB right?  Also please re-read my OP more carefully.  I am stating that flat base bullets specifically have an advantage over boat tail bullets at 100y all else being equal, which is true.  For 5.56/.223, there are actually surprisingly few flat base match grade bullets available, all three of the best of which I mentioned in my post.  The vast majority of 40-55gr ammo is not flat base, they have small boat tails and this includes the M193-style 55gr bullets used in the loads you have been shooting.  I am NOT stating that 55gr loads are inherently more accurate at 100y.  Just that a high quality match FLAT BASE bullet will be more accurate than a boat tail bullet at 100y, all else being equal.  Additionally, A 1:7 twist will stabilize basically all 55gr bullets, and some of the longer 50gr and 45gr bullets.  Once a bullet is stabilized properly (not under or over) twist rate does not effect accuracy, so the RDB will be just as accurate with a 55gr bullet as it will with a 77gr bullet, all else being equal (again if the 55gr has a flat base, it will be slightly better at 100y).

Hope that clears everything up for both of you.  I guess I wasn't clear enough in my OP on a topic I thought was very obvious.


Furthermore I would just like to add that high quality match ammo will be massively more accurate than normal use stuff in almost all cases.  But you must find the load that matches your rifle barrel's node to really take advantage of the massively increased consistency of match ammo.  This requires testing a lot of match loads in your gun unless you reload.  What I posted on the prior page was the data I had from my precision AR + Molons, and that should give you an idea of the number of loads that need consistent testing to find the true mechanical accuracy limit of the rifle (again if you don't precision handload, if you do you can just do a ladder test and refine your load from there).
« Last Edit: May 19, 2017, 03:48:57 PM by Potss » Logged
HBeretta
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« Reply #26 on: May 19, 2017, 05:12:01 PM »

Ok, a lot of misunderstands to clear up here.

2.) @ HBeretta, you realize you quoted BrianK and not me right?  And you realize BrianK is talking about the M16-->M16A2 progression, not the RDB right?  Also please re-read my OP more carefully.  I am stating that flat base bullets specifically have an advantage over boat tail bullets at 100y all else being equal, which is true.  For 5.56/.223, there are actually surprisingly few flat base match grade bullets available, all three of the best of which I mentioned in my post.  The vast majority of 40-55gr ammo is not flat base, they have small boat tails and this includes the M193-style 55gr bullets used in the loads you have been shooting.  I am NOT stating that 55gr loads are inherently more accurate at 100y.  Just that a high quality match FLAT BASE bullet will be more accurate than a boat tail bullet at 100y, all else being equal.  Additionally, A 1:7 twist will stabilize basically all 55gr bullets, and some of the longer 50gr and 45gr bullets.  Once a bullet is stabilized properly (not under or over) twist rate does not effect accuracy, so the RDB will be just as accurate with a 55gr bullet as it will with a 77gr bullet, all else being equal (again if the 55gr has a flat base, it will be slightly better at 100y).

Hope that clears everything up for both of you.  I guess I wasn't clear enough in my OP on a topic I thought was very obvious.


potss...yes i'm well aware that i quoted Brian - running with the progression of faster twist rates to accommodate heavier bullets going back to the 60s.  RDB or not - it's a 1:7 twist 17" barrel.  

now i get what you're getting at now that you've clarified 'once a bullet is stabilized properly'.  but to imply twist doesn't affect accuracy doesn't hold for me.  you can't go shooting 77gr through 1:14, 1:12...hell even 1:9 and not see accuracy issues in the form of bullet yaw/keyholing etc...the decreased twist rate will certainly result in a dramatic decrease in accuracy.  and yes i'm aware of flats vs boats.  BUT, again, you did clarify once stabilized.

and i'm not disagreeing with this notion of 55gr bullets.  i realize we could go in circles all day about why some guns shoot better with bullets of one weight versus others.  how heavier bullets will not remain stable over longer distances if they don't have enough velocity and so on.  i mean we could throw everything out the window and say barrel length, twist rate and a lot of other factors determine accuracy.  do i think bullet weight matters?  yes!  do i think it's the only reliable indicator for accuracy?  no, as i feel twist rate, velocity and barrel length plays into this.  

that or i could just throw semantics aside and go out and keep shooting various ammo types to get a better feel for what my RDB prefers.

i assumed my RDB would handle the heavier loads better and it has but like i mentioned hasn't been drastic and in fact some might consider negligible, but i'm the shooter...so subtleties are noticed.

i'm open to different perspectives and until you clarified, with you apparently dismissing twist rate altogether initially or so it seemed, i was definitely questioning your approach.  i'm still not on board with your statement about twist rate, but will look to your ammo recommendations to see what performance i get.  again, i'm not entirely convinced my RDB favors the heavier bullets, but like i mentioned previously - have noticed less deviation with them and best groups on those occasions.

in fairness, InRangeTV didn't really see much difference with their RDB going from 55gr to 69gr but then again they were shooting a pre-production 1:9 twist rifle.
« Last Edit: May 19, 2017, 05:18:18 PM by HBeretta » Logged
Potss
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« Reply #27 on: May 19, 2017, 08:10:44 PM »

I'm glad my updated post was more clear, but I can see it still wasn't entirely clear.  When I say "properly stabilized" I mean specifically the bullet is not under or over stabilized.  In other words, I don't mean a 77gr out of a 1:12 or a 40gr out of a 1:7, those are not stabilized properly.  Here is conclusive evidence that as long as you are in the stabilization range, twist does not effect accuracy: https://www.ar15.com/forums/t_3_16/687746_55_Grain_Bullets_Fired_From_AR_15s_with_1_7_Twist_Barrels.html  Better yet it is specific to 1:7 vs 1:9 and 55gr.

I am glad you are going to be testing more ammo types in your RDB, and I look forward to the results  Smiley
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