.25 cal 51 gr. NOE Bob's Boattails

[K.O.]

Believe what you want.... I'll follow the advice of Robert McCoy.... Those graphs are specifically for the 51 gr. BBT, calculated using his program, developed at the Aberdeen Proving Grounds.... If he says the stability when Subsonic decreases with increasing velocity, I believe him as to the trend.... How the actual SF number relates to the twist required may be up for debate, but IMO not the general principles.... You will find that general shape of graph for every projectile, complete with the discontinuity at Mach 1....

Bob

Bob with respect I will use your own words against you... but I had come to the conclusion on my own a while back after seeing the shadowgraphs in Mr Mccoys paper on .22lr ( BLR-MR-3877 ) and reading Robt Rinker, 'Understanding Firearm Ballistics, Basic To Advanced Ballistics Simplified & Explained,'  wish I still had it can only hold on to general ideas now (brain probs)

anyways with humility cause it is easy to be wrong but I do think this design accomplishes exactly what you describe... how is up for debate...

"As an airgunner, I deal in subsonic flight almost exclusively.... I think you will find that for most bullet shapes the minimum stability occurs at a couple of fps under the speed of sound.... The exception is a cylinder (flying ashcan) where the stability continues to drop as the velocity decreases.... Bullets with a very short nose and wide meplat can have a nearly constant stability at nearly all speeds subsonic, but you virtually have to design a bullet to act that way...."

from  http://castboolits.gunloads.com/archive/index.php/t-209192.html

remember our discussion of true hemi spherical vs flat nose or semi hemi pellets and that about 600 fps is the place where the wadcutters flow changes... sort of the same deal...

but way more complicated...



I really wish my brain worked better than it does now so I could present my thoughts in this discussion better...


                     

[K.O.]

Owen "Hot Dog" pretty much MOA at 50 and not so bad at at 121 unsized...

It is hard to tell but it does look like just a bit of yaw at 120... might go away (if it is even there pics can deceive) when the barrel is not doing the sizing...

Makes me want to steroid my Mrod now instead of later..

[rsterne]

Kirby, yep, the part you put in bold pretty much sums it up.... but that doesn't describe the 51 gr. BBT.... One look at the stability curves shows you that.... Did you by any chance look at the date of that post?.... I hope I have learned a little in the last 3 years.... I only started on the BBT quest a couple of years ago.... it just seems longer.... 

Until I find a better tool, I will continue to use Kolbe's Twist Calculator, which uses Robert McCoy's program to do the calculations.... All my BBTs, including the really short ones, show less stability as you increase the velocity in the subsonic range, according to those calculations.... I will continue to believe that is correct until somebody proves otherwise, it just seems the prudent thing to do....

Owen, there is no separate shell holder in my Lee press, the NOE rams fit right into the Lee holder.... I am still using my Lee rams.... I use a Lee bench press, but it should be the same?....

Bob

[owen]

Thank you, Bob. I haven't had it out it's box since 1998. I'll look into it, now.

Kirby, thank you for your input about the yaw. I don't know, yet. That target is a used UnitedStates Priority Postal mailing box that has sat at 121 yards for the last couple of months. The wind blows it over often. It rained on it the night before I shot that 3-shot, 2.196" group. The box wasn't perfectly square with my shooting position either. That 2-shots landed in the 1.375" circle was quite pleasing as I had only zeroed the gun at 50 yards. My phone app., Strelok Pro, told me where to hold at 121 yrds. and these are the very first 3-shots that I took at that target. I could be wrong, but I'm expecting great things after sizing.

[K.O.]

 "it just seems the prudent thing to do...."   I tend to agree that any thing about how it builds  the stagnation zone thru what speeds is conjecture and also tend agree about the general trend;

but can there be a period where the round sort of goes to sleep..?

with today's tech I wonder if we could observe the building /shape  of the stagnation zone at differing fps... and actually see physical  drag profile that would tell a lot...

How did you jigger the program for the rebated boat tail..? just use a steeper angle??? or just go by length and base diam... neither seems quite right..?

oh and in my defense Eley Tenex is sold as the nose being the reason for extra stability and I drank the cool-aid...




here are the clean versions of some of the pics in memo BRL-MR-3877

http://www.network54.com/Forum/79537/thread/1268775273/Changes+in+BC+with+velocity%85%85.continued



"
The iconic flat nose projectile of ELEY tenex is one of many innovations pioneered by ELEY engineers. As it cuts through the air its revolutionary design pulls the centre of pressure forwards, aerodynamically stabilizing the projectile and increasing accuracy at the target.
"
from

http://www.eley.co.uk/eley-tenex


    ?



Owen did you polish the barrel or does it just have some rounds thru it...

I need to take some measurement but it looks like the rifling is just a bit taller than the Benji rifling...other than that I think I  get about .2495 groove for the choke... looks a lot like  the Benji/ Green mountain and NPXL rifling.... will ad muzzle pic of the cut off you sent to post #79





 I was thinking about fpe and such being close to .22 target and thought...

in very good indoor conditions .22lr and a high bar but should be... so yep pulling off sub moa does look very possible...

http://www.accurateshooter.com/guns-of-week/22lr-rimfire-ammo-comparison-test/

[owen]

"Owen did you polish the barrel or does it just have some rounds thru it..."

If you're asking about the choke... neither. I cut the choke off before I ever fired a single shot through the brand-new barrel. I have polished the bore of the remaining 16.25" of barrel, though.

[T3PRanch]

I found a post a few pages back interesting concerning the AirForce .25 barrel as the following quote is direct from Rachel Gonzales at AirForce when I ask if the LW barrel listed on LW's site is the same as the Airforce LW barrel. AirForce's replied specs do not agree with the specifications posted  a few pages back:

"Hi Mr. Moore,

Our .25cal barrel measures .248 land and .255 groove.  Just about the same.
Twist rate 1:17.22.  Have a good day!"

The reply from AF was on 25 March 2016

Thurmond

[K.O.]

well I wish she would post that here and explain how a major re-seller post incorrect specs without them correcting the twist rate?



as for the bore /groove specs that might be just past chamber groove and bore at the choke  the lands are not that tall .007... looks more like  .0025+ to me... but hard to tell without a good blade mic.. maybe .004

[K.O.]

Well  Owen just trying to figure out these things even to the naked eye the rifling seems sharper  and the barrel smoother on the L.W. and it makes me wonder about the machinability of the alloy of each...the L.W. could be smoother because it is a touch softer(sulfer?)... at the price point of each I wonder how long each button is considered to be in spec...

wish that some reliable test would let us know more about the alloys of each...

can pretty much tell the L.W. is a swaged choke...

[owen]

"Our .25cal barrel measures .248 land and .255 groove.  Just about the same.
Twist rate 1:17.22.  Have a good day!"

Ok, I'll go slug my barrel and let you know what I find. I hope that this is a newly implemented specification and that my barrel is not like this, though.

Kirby, yes the LW barrel is softer than some others that I've lapped. I had one chinese, Crosman barrel that was softer, but another Chinese, Crosman barrel that was harder. A Turkish, Hatsan, barrel was the hardest that I've seen, so far. If what "Tritium" is saying is true then I'll probably be looking into buying the 1-16" Green Mountain barrel, so that I can use this mould and all of the resizing components that I purchased. We'll see soon.

[owen]

Unfortunately what "Tritium" has shown us about the Airforce barrel's ROT and bore dia. is true. My barrel mics exactly .255" dia. and the ROT is really close, if not exactly, 1 turn in 17.22 inches. I have never once doubted the advertised 1-16" ROT of twist that Pyramyd-Air, and others, have touted the Airforce gun's barrels to have. Now. I wish that I'd have gone with the 41 gr NOE BBT's, when they finally appear,  for this barrel. No use crying over spilled milk though, I'll immediately start looking into the GM barrel, which Rsterne, Bob, says has a 1-16" rate of twist. I want sub-moa accuracy with these bullets. By the way my gun, when shooting between 1008 fps and 1026 fps, shot the H&N Grizzly bullets, pellets into 3-shot, 3/8" groups at 50 yards, consistantly. At 1039 fps the group size was 1/2"-5/8", at 50 yards. 1100 fps would open group size to 1 1/8" and at 1123 fps group size grew to 3 1/2".

[owen]

Kirby, those photos are great. Thanks for posting them.

[rsterne]

I have never measured the MRod .25 cal GM barrel, but the specs I have seen, confirmed by some on this forum, is a 15.5" twist rate....

Bob

[dyotat100]

If your looking to change the barrel why not go to the 25 auto TJ barrel. 1:14 .250"

[owen]

"If your looking to change the barrel why not go to the 25 auto TJ barrel. 1:14 .250"

Maybe that's where I'll end up, but I want to try the GM barrel first.

[owen]

The NOE sizing stuff showed up yesterday, but I had other commitments and didn't get to the shooting bench until this evening. I had prepared about 450 bullets for sizing, over the weekend. These were tumble lubed with straight Johnson Paste Wax and then dried. I resized about 100 bullets through the .256" bushing. I then put them all back in the glass mason jar and tumble lubed them again with the residue of wax still left in the jar from the first lube. I then let these dry, on waxed paper, again. This evening there was still enough light left to chronograph some 3-shot strings. I adjusted the power setting until I was satisfied with the output curve. I ended up with a 3-shot average of 951.3333333 fps. 9 of these bullets weigh an average of 52.77777778 gr apiece. Average energy for 3-shots, with the guns present settings, is 106.1 fpe. I had company show up and after they left it was really too dark to start shooting groups, but I did shoot one group, at 50 yards. I didn't get the caliper out, but I could see it was right at 5/8", for 3-shots. That's actually quite encouraging as even when I had this gun, with another projectile, shooting 3/8" 50 yard groups, in this same lowlight I'd usually get 3/4-1" groups.

[dyotat100]

Not sure about lubing your bullets. I use imperial sizing wax and only put a little on my finger to rub on a bullet. I do that about every 10 bullets to make sizing easy.

Also what size is your barrel?  .256" seems awful large for a .25

I found your barrel size. Try shooting smaller. .001" in my opinion is to much. I don't shoot more then .0002".

Did you check the bullets after sized?

[owen]

Not sure about lubing your bullets. I use imperial sizing wax and only put a little on my finger to rub on a bullet. I do that about every 10 bullets to make sizing easy.

Also what size is your barrel?  .256" seems awful large for a .25

I found your barrel size. Try shooting smaller. .001" in my opinion is to much. I don't shoot more then .0002".

Did you check the bullets after sized?

Basically, I got into this NOE 51 gr BBT bullet casting thing pretty much not knowing anything about bullet-size/bore-size relationships. Thankfully, Bob steered me right, or I'd have bought the wrong mould size. At the time that I ordered the sizing components, last week, I was believing that my barrel bore was .254" and that it had a 1-16" twist. So I ordered 3-sizing bushings, along with the universal die, from NOE. The sizes that I ordered are .254", .255" and .256". After placing my order, I also spent a whole lot of time reading various forum threads on both the subject of bullet-lubrication and bullet-size/bore-size choices. One thing that really stood out was that it was stated that undersized projectiles is the no. 1 cause for barrel-leading. Also an oldtimer, that has been lubing his rifle bullets with Johnson Paste Wax, for over 30 years, then in 2013, went on to say that most people feel safe from barrel-leading by shooting .001" over bore-size bullets However, he said that because of manufacturing inconsistancies in the length of a bore that he believes that .002" over bore-size is better choice. With all of this information I had already determined that I was only going to use the .256" sizing bushing. Then I found out that by bore is not even .254", but rather .255" and I have been agonizing over that ever since. And now you're suggesting that under .001" is better still. So, I have no idea who is right or wrong or why exactly.
No, didn't measure the bullets after sizing. I'll need to push a bullet through the sizer and measure it before I add wax. If I remember then I'll do that tomorrow, LW, and I'll let you know what I find.

[K.O.]

I have been thinking that If I want to use a lube I thought I gotta eat what I lube with... so I am thinkin bees wax and canola or olive oil or even tallow..

good enough for Eley.

[dyotat100]

Who says barrel leading is from under size bullets?
30 years must be PB. What works for PB is not the same for air.

We are dealing with 3000 psi and not 50,000 psi. Shooting that much over size is just wasting fps. I know shooting a .0005" larger then needed is about 5' at 400 yds lower.

When getting groups the size of a asprin or barely larger at 100 yds using bore sized slugs or .0002 " under is not proof enough to what works then I don't know.