Improved BBT ?

[rsterne]

I have been giving a lot of thought to redesigning some of my Bob's Boattails.... Miles Morris has done quite a bit of work on optimizing the nose of airgun slugs on a theoretical basis.... His research and calculations show that using a Meplat of about 50% of the caliber is a good choice for velocities up to Mach 0.9.... Going smaller does not have a benefit in that speed range, but going larger does increase the drag throughout all velocities.... The drag increases rapidly above 1000 fps, even for quite small Meplats.... This 50% Meplat is consistent with the Corbin dies, and many of the commercially available slug designs, which are also swaged.... Part of the reason is that going to a smaller ejector pin than half the caliber, the pin is prone to breakage, particularly for HP pins....

Miles has also looked at the angle of my boattail designs, and has suggested that anything above an 8 degree taper may be seeing the flow detaching from the boattail.... This is made worse by the rebate, and Miles has told me that the angle from the back of the drive band, where the rebate starts, to the base of the bullet should probably be not more than 8 degrees.... This means that for the rather short boattails required so that airgun slugs still have a good supported length in the bore, the boattail itself has to be quite shallow, on the order of 5 degrees.... and that the rebate step should be kept to a mimimum.... Miles also suggested that a shallower ramp in front of the drive band may show a small drag reduction as well....

I really like the slightly reduced midbody diameter of my BBT's because keeping that part of the slug just a couple of thou smaller than the land diameter reduces the bore contact, and makes a big difference to the bore drag.... The small step immediately behind the nose should "trip" the boundary layer to turbulent, and since turbulent boundary layers tend to stay attached better than laminar ones, this feature has a double benefit.... I reasoned that if I make the rebate only the same depth as that front step, the boattail would start cleanly, undisturbed by the rifling lands, yet the drive band would be molded square to the bore, giving the clean release from the muzzle which is one of the major reasons for using a rebated boattail.... Unfortunately, the Corbin rebated boattail dies require a 0.015" step just so that the die does not break, but we are not limited by that consideration in a mould for casting slugs.... On my original BBT designs I mistakenly assumed that a 0.015" step was the optimum, rather than as shallow as possible while still working to provide a clean release from the crown....

I borrowed the small radius at the edge of the Meplat from the "Ranch Dog" designs on the NOE website.... This feature is claimed to reduce the nose drag by reducing the turbulence where the flow transitions from the Meplat to the Nose Ogive.... and that certainly applies to Supersonic flow.... IMO, it stands to reason it can't hurt at airgun velocities, and it allows me to decrease the flat diameter of the Meplat by about 10% (down to only 50%) without lengthening the nose for a given Ogive radius.... The Meplat on this design is significantly smaller than my previous BBT of this length, which was 70%.... These are the changes I have incorporated into the drawing below....



All the dimensions are scaled to the caliber of the slug, so the design can be scaled up and down within the range from .172 to .308 cal.... The twist rate would also scale proportionally to the caliber.... The weights for those other calibers are shown in blue.... The less pronounced boattail dimensions, when put into the Kolbe Twist Calculator tell me that this design should work in a slower twist than the previous BBT's of this length, being more similar to a flat based bullet because the base is larger in diameter.... The length supported in the bore is virtually a full caliber long, because the Ogive touches the lands where the vertical dotted line is, holding the slug centered in the bore.... and incidently allowing for easier chambering because the Ogive is a very shallow slope at that point....

While the boattail base is not as small, the other changes should have made the drag similar to or less than my previous 2.0 caliber long BBT designs.... Miles feels that maintaining flow attachment over the boattail is very important to accuracy, so hopefully that will be improved as well.... This design could not be made by Accurate as the Meplat is too small (even in .308 cal), but it could be made by Arsenal in any of the sizes shown, but of course they don't make HP moulds.... With NOE being completely overwhelmed at present, I haven't even suggested to Al that he consider making these so that we can try them.... However, if anyone wishes to have a mould made, please feel free to contact me for a dimensioned drawing for the caliber of your choice....

Bob

[Mr.P]

These look great I'd love to get a mold for these projectiles.  Need to read your posts again to make sure I understand what it is you're saying though.

Enjoying the old boat tail molds as well.

[rsterne]

I concentrated on this length (2 calibers long) as it covered the greatest variety of PCPs with one design that could be easily scaled up or down.... I am hopeful that some moulds will be made in various calibers so that I can use that experience to look at shorter (lighter) and longer (heavier) designs for the calibers appropriate to them....

Bob

[Greg_E]

I'd be in for a .357 mould if a manufacturer could be sourced, they all seem to be overwhelmed with business right now. Anyone that can make stuff for popular firearms calibers seems to be backed up for months.

Longest that will fit a Bulldog mag is .740 inches. The 150 grain Griffin boat tail slugs are the biggest that fit and still revolve.

[sb327]

The fact this popped up about the time I’m getting my .257 going good is kind of a sign I need to make a mold before making a swage die.

I am really liking the power the 70 grain is making at the 1000fps range. So, I’ve got a question. I know you would like to prove the design at common pcp length first but what aspects would change if you lengthen this about 10 grains worth?

I suppose I could make a 61 grain as well so I would have a couple of choices.

Thanks for sharing your work on this,
Dave

[rsterne]

This design scaled up to .357 cal would be 175 gr. for the FN and 165 gr. for the HP.... That's pretty heavy for most .357's.... If you can launch that heavy a slug, it would be 0.714" long in .357 cal.... However, if you left all the lengths the same, but scaled up the diameters, it would weigh 126 gr. for the FN and 119 gr. for the HP.... but the angle of the BT would increase, and it would end up outside the 8 deg. suggested by Miles.... In reality, going from this design all the way to .357 cal is probably not the best idea, a shorter design is required for more common slug weights in .357 and .457 calibers.... with the appropriate design changes made.... A length of 1.5 calibers, with all the changes optimized, would likely be a better choice.... It would not end up with much of a boattail, however....

Conversely increasing the length to add 10 gr. to a .257 cal would again change all the features, and a better solution would be to make a new design to optimize it at, say, 2.5 calibers in length, and something around 75-80 gr.... That is one of the things I want to do, as time permits, but I wanted to start with this 2.0 caliber long design, as it covered many of the popular calibers....  Eventually I want to make a 3.0 caliber long version as well, and maybe even longer for the small calibers....

Since the Sectional Density changes in proportion to caliber when you scale a slug design up and down, what happens is that as you go larger, the required pressure and barrel length increase.... Going the other way, the SD decreases, so you can use a longer slug design without having to run excess pressures.... That is exactly why a given length in calibers won't work for all calibers....

Bob

[rsterne]

Dave, I have drawn up a .257 cal. that is 2.5 calibers long, it weighs 74 gr. as a HP and 78 gr. as a FN.... and the good news is that it should work fine in a 14" twist, which is the most common TJ's barrel....



With the Meplat reduced to 50%, I didn't need to radius the front edges, and doing so would have limited the size of HP that could fit.... As before, weights for .224 to .308 cal are shown in blue....

Bob

[sb327]

Bob,

Thank you for taking the time to draw that up. I did not expect a drawing, I really do appreciate the time you put into it.

I will probably try to whittle out a cutter today and maybe get a mold block ready to cut. But I’ll be heading back to work in the am, so it’ll be a couple weeks after that.

Thanks again,
Dave

[Greg_E]

Thanks, yes that would be pretty heavy in .357. I can see why so many convert to .257.

[YEMX]

Probably a silly question- but if this design were to be done in .243, what would the OAL of the slug be? 

[triggertreat]

Thanks, Bob!  I am glad to see the BBTs taking on some refinements.  I would certainly be interested in a 55 grain or less mold in a  .30 cal version for airguns.

[rsterne]

Tom, the 2.0 cal long would be 0.486" long and the 2.5 cal long would be 0.608" long in .243.... just multiply the length times the caliber....

Keith, trying to incorporate a boattail in a 55 gr. slug in .30 cal would be nearly impossible, particularly with trying to keep the base within an 8 degree angle from the back of the drive band.... Even a gas-check stub might be difficult with that short a slug.... My existing 67 gr. BBT is only 1.25 calibers long.... I would suggest a flat-base slug is your best bet.... Scaling down the 88 gr. NOE .357 cal, or something similar, would be your best bet.... It would be 52 gr. as a FN, and 49 gr. as a HP....

Bob

[triggertreat]

Tom, the 2.0 cal long would be 0.486" long and the 2.5 cal long would be 0.608" long in .243.... just multiply the length times the caliber....

Keith, trying to incorporate a boattail in a 55 gr. slug in .30 cal would be nearly impossible, particularly with trying to keep the base within an 8 degree angle from the back of the drive band.... Even a gas-check stub might be difficult with that short a slug.... My existing 67 gr. BBT is only 1.25 calibers long.... I would suggest a flat-base slug is your best bet.... Scaling down the 88 gr. NOE .357 cal, or something similar, would be your best bet.... It would be 52 gr. as a FN, and 49 gr. as a HP....

Bob

Thanks Bob!

[Greg_E]

I'm going to post this, because I don't have the knowledge to understand why or why not on this question.

Back to the problem of weight. Would it be possible to use a deep hollow tail vs hollow point? I'm sure there are reasons this isn't done, but I don't know them.

[rsterne]

You can do that, but you have to make a nose-pour mould, like the NOE pellet moulds.... It moves the CG further forward, which is good for a flare-stabilized projectile like a Diabolo pellet, but sometimes not so good for a spin stabilized slug.... You would have to try it on a given design to see how it works.... Some shotgun slugs are made that way, and a few "mini-balls" for muzzle loaders....

Bob

[rsterne]

Here is the improved BBT design for slugs that are 1.5 callibers long.... I have used all the improvements listed in the OP, within the limitations required for such a short slug.... Notice that the boattail has all but disappeared.... Once again, the weights for other calibers are shown in blue....



Note that these calibers are typical "airgun" calibers instead of the PB calibers I used above.... as these slugs in the smaller sizes (up to .30 cal) would be more likely to be used in airgun barrels.... The slug weights in .357 and .457 caliber are much more typical of what we might use for those.... Once again, the twist rate is proportional to the caliber.... so the .30 cal slug would be optimal in a 26" twist.... 

Bob

[YEMX]

Hmmm...  Would a 1.5 caliber long .243 be possible?

[rsterne]

Absolutely, Tom, in .243 caliber it would weigh 40-42 gr....

Here is the next of the new designs, a 3.0 caliber long version....  Once again, the weights for other calibers are shown in blue, and the twist rate is proportional to caliber....



These slugs are starting to push what twist rates are available, and especially in the larger calibers will require high pressures and long barrel lengths to get into the 900s.... If a larger HP is desired, the Meplat could be pushed back to the red dotted line, until it is 50% (it is drawn at 40%).... That would shorten the slug and reduce the weight a fraction, without much if any increase in drag, according to Miles' calculations....

Bob

[mackeral5]

This design scaled up to .357 cal would be 175 gr. for the FN and 165 gr. for the HP.... That's pretty heavy for most .357's.... If you can launch that heavy a slug, it would be 0.714" long in .357 cal.... However, if you left all the lengths the same, but scaled up the diameters, it would weigh 126 gr. for the FN and 119 gr. for the HP.... but the angle of the BT would increase, and it would end up outside the 8 deg. suggested by Miles.... In reality, going from this design all the way to .357 cal is probably not the best idea, a shorter design is required for more common slug weights in .357 and .457 calibers.... with the appropriate design changes made.... A length of 1.5 calibers, with all the changes optimized, would likely be a better choice.... It would not end up with much of a boattail, however....

Conversely increasing the length to add 10 gr. to a .257 cal would again change all the features, and a better solution would be to make a new design to optimize it at, say, 2.5 calibers in length, and something around 75-80 gr.... That is one of the things I want to do, as time permits, but I wanted to start with this 2.0 caliber long design, as it covered many of the popular calibers....  Eventually I want to make a 3.0 caliber long version as well, and maybe even longer for the small calibers....

Since the Sectional Density changes in proportion to caliber when you scale a slug design up and down, what happens is that as you go larger, the required pressure and barrel length increase.... Going the other way, the SD decreases, so you can use a longer slug design without having to run excess pressures.... That is exactly why a given length in calibers won't work for all calibers....

Bob

The 2.0 caliber .357 is interesting, too bad we will likely never see it available in a hollowpoint.  Arsenal would make the solid based off a drawing, correct? Texans, modified Bulldogs, Extremes, XP's....lots of guns could make use of this one. 

[rsterne]

If NOE ever get caught up and are looking for new designs, then it would be possible.... Otherwise you would have to have it made by Arsenal, and then send it to Erik at HP Mold Services to have the HP system installed.... If the radiused corners at the Meplat were reduced, then Accurate would be able to make it as well (they have a 0.180" minimum diameter for the Meplat).... but again, no in-house hollowpoints.... I can supply you a dimensioned drawing for the FN  version of the 2.0 cal. long in .357, for either company, if you want one....

Bob