Why are Lube Grooves the way they are?

[GabbyM]

Thank you all. That's an interesting and informative insight.

OK, that was a surprise. I was thinking shallow and narrow grooves would be better for pan lubing.

I'm also surprised that wider bands make little or no difference. I was expecting deep and wide grooves to collapse easier, especially if close to the boolit base. I have observed lube being left in the grooves which may or may not get thrown out in flight.

What about the shape of the lube groove? Does it make any difference if they are square or angled? Square would increase the bearing area and would hold lube in better (as in preventing it from flinging out) but then angled grooves would fling it out more reliably.

Square lube groves hold lube tighter. But are harder to fill out when casting. Plus don't drop from mold as easily as rounded groves. Dropping from a mold is the biggie. Skinny o ring sized channels of lube can tend to yank right off a bullet. That's why this forum has an entire category devoted to lube recopies. Add some lanolin or any of the other dozens of fixes for making a more sticky lube. All part of what keeps this site jumping.

[303Guy]

Good point there GabbyM. I'm planning to make moulds for myself using a groove cutting type boring bar so that would not be an issue. That's basically why the question came up. I must confess the Loverin design has a lot of appeal and is in fact what I'm planning on emulating. My plan is to step the boolit down in stages to fit the throat profile. But I need (or want) an understanding of why things are done the way they are and users are probably the best source of information (especially on this site where the knowledge, experience and logic base is pretty damn good).

[GabbyM]

Thanks for this.
I have asked this question in other places, and since this has begun anew, may I ask, how/what determines gas groove design?
Agreed, the mould manufacturer has to cater to a large variety of usage patterns ( and keeping economics as a priority) and lube.if one were designing a CB, for say a .309" or .286" smokeless load, to run with, say, NRA 50-50 or Ben's Red specifically, would the design parameters change so drastically? Yep, you guessed it, I'm a lube noob, and find it very therapeutic, making up lubes.
Cheers and TIA

Much of that simply is dependent upon what fits. So often things come in two sizes. To big and to small. Say your bullet body has a length of x. You want a lube grove width of y. Then you find 2y doesn't fit in x but 1y is not enough. So you end up with y+z. now and then xyz come together to make a fine looking boolit. It sells like hotcakes for a century no matter if it shoots for a hoot or not. About like a Ford Mustang. Things don't drive for nothing. But are perceived as cool.

[GabbyM]

Good point there GabbyM. I'm planning to make moulds for myself using a groove cutting type boring bar so that would not be an issue. That's basically why the question came up. I must confess the Loverin design has a lot of appeal and is in fact what I'm planning on emulating. My plan is to step the boolit down in stages to fit the throat profile. But I need (or want) an understanding of why things are done the way they are and users are probably the best source of information (especially on this site where the knowledge, experience and logic base is pretty damn good).

I’ve three Loverin style boolits. They are stubborn to drop from a mold. Simply due to all those groves. Tapered groves help immensely. Groves smaller at bottom than at surface. Thus a simple off the shelf circle saw on mandrel comes short. You end up needing a tapered cutter costing a pile of money or you grind it yourself to find the week is over and you’ve not put your product out the door yet.

The old style cherry cutter is hard to beat for a small shop with a knee mill. Thing is you can’t put some minimum wage fool or drunk relative on the machine then expect a cutter that takes a day to make to last more than fifteen minutes. If that long. Which is exactly why you don’t see bullet mold manufactures popping up like McDonalds drive ins.

About any tool grinding shop now days can make you a cherry cutter. Anything you can draw up on a CAD then send over the phone line. But there is that bill they send. Six axis CNC grinders don't come cheep.

[GabbyM]

If all you want to do is cut a mold now and then. Running them on a lath. Especially a single cavity that doesn't have to match up to a sister cavity. Is the easiest way to go. Just grind a set of bore bars and go with it. making multi cavity molds on a lathe is done with CNC lathes. Still not easy.

[303Guy]

Thanks, GabbyM. I'm always interested in how things are made. I haven't spent enough time on a lathe to become bored with it. Nor a milling machine. I have a lathe at home but only a small one (the largest bench top available). So, the more information the better. I won't be considering a multi cavity mold any time soon.

[monkeywrench]

If Elmer Keith said it. I believe it. That's final.

[JSnover]

Thanks for this.
I have asked this question in other places, and since this has begun anew, may I ask, how/what determines gas groove design?
Agreed, the mould manufacturer has to cater to a large variety of usage patterns ( and keeping economics as a priority) and lube.if one were designing a CB, for say a .309" or .286" smokeless load, to run with, say, NRA 50-50 or Ben's Red specifically, would the design parameters change so drastically? Yep, you guessed it, I'm a lube noob, and find it very therapeutic, making up lubes.
Cheers and TIA

Look up Mountain Molds (there are others but I designed my favorite mold using their site). Send them your question and please share the answer with us, I think it would be interesting to hear from a custom mold shop, what works and why.

[Mal Paso]

The current version of Lymans 429421 has a groove designed by the lubrication sales department. The 503 clone (also Elmer's) I shoot now takes a Lot less and I still get a good lube star at the crown.

Don't think Elmer intended it to have a reduced front band either.

[fredj338]

IMO, the less diruption you have in the bearing surface, the more accurate the bullet is likely to be. The grooves have to be wide & deep enough to hold a lube & the lube type could determine the depth of the groove. Why I prefer a single groove large enough to hold wax based lubes.

[nanuk]

Keep in mind the grooves have to carry enough lube for the trip down the barrel weather its a 16" or 34" length,....

But if the boolit doesn't collapse and force the loob to the barrel, all you are doing is carrying loob out the bore, and flinging it into the air.

and if the barrel is smooth, VERY LITTLE loob would be needed to seal the boolit, as the seal is pulled with the boolit....


I dunno, but it is interesting that some have said square, or flat bottomed grooves make a more accurate boolit than rounded grooves.

perhaps is it just magic??

[detox]

My most accurate .357 Magnum bullet tested has two shallow round cut grooves. RCBS 147gr 9mm mould #82077

[BABore]

A lube groove on a well designed boolit is more than just an afterthought. There are a multitude of things to consider. What is the boolits intended purpose, velocity range, bearing length, bearing surface, firearm consideration, and on and on. The shape, depth, width, and positioning of the groove(s) will determine how it distributes lube and even more importantly how you allow or not allow the boolit to upset or obturate.

Any 1st year engineering or architecture student knows about columns and their strengths. Rounded lube grooves offer the strong support of the Byzantine arch. Even a thinner base band will resist obturation with this type of groove. Lube pumping action is more restricted as well. Keith designed his SWC's with a deep, square groove for the opposite affect. The groove is more easily collasped to pump lube and allow the base to bump up to fit the large throats of his Colts (my opinion). The square cornered, flat groove is the weakest design in grooves and support columns. When you start angling the sides of the groove, you add strength and let the boolit release from the mold easier. The more angle, the stronger and easier it gets. If you want angled sides, but also want the groove to collaspe more readily, then you use groove depth as an aid. IMO the main reason you see rounded lube grooves or the flat bottom groove with a 25 to 45 degree per side angle is cutting tool integrity. The same goes for mold makers that limit the depth of the groove. It's not that they are experts in lube groove design. They just don't like breaking expensive tools inside mold blocks.

The depth, width, and placement of lube grooves can and are used to affect the balance and center of gravity of a boolit. For every boolit diameter and desired weight there is a bearing length that has to be considered. The rest is the non bearing portion of the nose and any bore riding length if it applies. Yes, a bore ride section is bearing length in terms of support in the barrel, but it doesn't require the same lube as driving bands. Based on the velocity range, barrel length and bearing surface of the boolit, you need a certain lube capacity. Noticed I switched terminolgy from bearing length to bearing surface. Bearing length is the total length including grooves. Surface is only the bands in contact with the barrel, added together. The lube capacity is based on the groove width and depth. Where you put that groove or grooves along the bearing length affect the balance and also driving band performance.

Quite a few years back I did some analysis on the original Keith boolit dimension. I modeled all of his semi wadcutter designs as solid models. All of the different calibers have almost identical lube capacities based on bearing surface. Bearing length as a percentage of OAL and CofG are also nearly identical. Not an accident I think.

The number of lube grooves can play an important part in the design and resulting accuracy. Smaller handgun boolits and certain rifle designs (silhoutte type) usually only have a single groove because that's all that will fit. Longer, heavier handgun, and more traditional rifle designs usually benefit from smaller multiple lube grooves. The lube capacity needs to be considered as usual, but you can error on the high side with mulitple grooves. You can choose not to lube all the grooves if it's not required, or you lube them all with a lube of lesser quality. The big thing is you have options. Once the base band, front band, and crimp groove (if required) are determined, the remainder of the bearing length can be made up of the desired lube grooves (capacity) and driving bands. They don't have to all be of equal lengths and usually are not if you are trying to juggle a proper CofG for the design.

When I was making molds I decided to run a little test with a 458 boolit design. I made up two 0.460 dia., 420 grain GC designs with a tapered bore ride nose. I turned the cherry's one right after the other and recorded all my setup's. Both designs had the same bearing lengths, bearing surface, lube capacity, crimp groove, etc. The only difference was the number of lube grooves and driving bands. One had a single big groove and two wide driving bands, the other had two grooves and three bands over the same exact lengths. The CofG's were both within a couple thousandths of each other. Cast from the same alloy they were within two grains of each other. I sized and lubed them with the same lube and worked up loads for both using multiple powders. With every load, the multiple lube grooved boolit out shot the single grooved boolit. 100 yard, 5-shot groups consistently ran 0.50" larger. Only one gun in one caliber, but worthwhile non the less.

[geargnasher]

Bruce, I have a question that has bugged me for years. I have a 35-220 and have seen excellent photos of the .30-silhouette. Both appear to have a small nick in the cutter in the center of the lube groove which leaves a slightly raised ring dead-center of the groove on the boolits. Is this intentional for the purpose of creating a more uniform crush area, as with other types of engineered, cylindrical crush zones?

Gear

[soldierbilly1]

My boolit's groove is bigger than your boolit's groove! Nah na na na nah na na na nah na....

prs

ahhh. groove envy. very pervasive billy boy

[felix]

The more weight on the outside of the boolit, the better. As well, the less weight on the inside of the boolit, the better. A general rule: Hollow points only shoot mo'better than boat tails only. That's why wheels prefer to have spokes instead of a solid mass. Therefore, multi SHALLOW lube grooves are most desirable whenever possible. ... felix

[cwheel]

I'm getting quite the education here, very timely discussion. Going to try to make my first set of mold blocks here, cut a cherry, make my first mold. My mill has set idle for almost 6 months now, and I can't seam to find a 38/.357 mold that I want to my next run with. The lube grove thing is one of my questions as well, the preceeding posts have answered my questions to the point I'm ready to give it a go. Single cavity first to test the design, 6 cavity once ( if ) the bugs are worked out. Thanks to the OP and all those who answered.
Chris

[Oreo]

A few things I haven't seen mentioned yet that I came across while researching the design for the 10mm wfn (Mihec group buy) boolit I designed:

When the boolit exits the muzzle ideally it should shed all the lube immediately, or carry it all the way to the target. Shedding being preferred. If the lube half sheds then the boolit is destabilized somewhat and accuracy suffers. Even if the lube is retained it may not be as perfectly balanced as shedding the lube completely thus the preference. So, you design for that but as was mentioned in previous posts there is some trade off because you want a boolit that will retain the lube in the groove till it is loaded.

Other considerations are that Elmer Keith tried a 90° square grease groove but the mold wouldn't release the boolits. There needs to be at least some draft angle.

Also, it seems that there are more forces pumping the lube then just compression of the groove. There is centrifugal force from the rifling as well as the acceleration of the boolit causing the lube to slosh toward the rear of the boolit. The gasses that blow by the boolit before the boolit is fully engraved in the rifling may also carry some lube forward of the boolit.

And finally, in my research I came across some mention of poor lube groove design causing diesel combustion of the lube (which was bad for accuracy.) This was mentioned in regards to lube grooves that were too deep and that the lube groove only needed to be just a hair deeper then the lands of the rifling.

[44man]

I don't like to horn in on anyone's business but I also deal in lube grooves. Postage is very cheap sending them!
I HATE one large groove just from an accuracy standpoint in my revolvers. Something else is going on I can't put my finger on and it might be balance.
I started to make two to three grooves in my home made molds and even though they carry the same amount as one big groove and the drive areas are also the same in total, they are much more accurate.
TL? One of my best is the RD 265 gr in the .44 but I refuse to unbless it with any TL. I use Felix and the bugger will hold 1-1/4" at 100 yards from my old SBH.
The Keith with the very huge groove will not go in my revolvers any more.

[390ish]

The only Loverin style mould I shoot is a Lyman 160 grain 8mm. I must say it performs better than just about any other rifle mould I own. I switch up between pan lube and Alox. The only difference I. An tell is the amount of smoke. I end up with Alox on the bottoms of gas checks and that makes for a bit of the smokey stuff upon ignition. I need to find a few loads to work up in 7.62x54 for Lyman 311284. It shot so well with full tilt loads out of a Krag. I am lucky enough to be able to shoot my surplus rifles at 150 yards and like to generate velocity to a degree that I am not too far from the distance shown on the iron sights when shooting. The 311284 has about as square of grooves as I have seen on a rifle mould.