SHERMAN MEGA LINE

[highdrum]

@brittf go long fb. I know one the best benchest smiths in the country runs 250fb with 180hyb in a 7saum. They all shoot lignts out with a lot less neck engagement. Yes the smallest raw accuracy is 95% the time found at lands or 40k less or jump. Other jump nodes further back, but usually don't shoot as small.

 

[brittf]

Thank you both for your replies.

I suspect that close to 'touch' (let's say jump < 0.020") is probably best for accuracy nodes. My drawings suggest that an extra 0.100" might be a tad too much additional Freebore (it takes total Freebore from 0.205" to 0.305"). As 'highdrum' states, his gunsmith goes with 0.250" Freebore with the 180 gr Hybrids in a 7 SAUM for a jump of <0.040" which makes sense to me.

In fact, I would say that 0.250" Freebore might be a sweet spot for the 7mm Sherman Mega as well, but that 0.305" is pushing it.

Let me give you some numbers for the 7mm Sherman Mega with the Berger 180 gr VLD bullet:

• The standard chamber has the bearing surface end 0.084" inside the neck and 0.249" bearing engagement in the neck at touch
• The long chamber has the bearing surface end 0.184" inside the neck and 0.149" bearing engagement in the neck at touch

Here are some more numbers for the 7mm Sherman Mega with the Berger 190 gr Long Range Hybrid Target bullet:

• The standard chamber has the bearing surface end 0.021" inside the neck and 0.312" bearing engagement in the neck at touch
• The long chamber has the bearing surface end end 0.120" inside the neck and 0.213" bearing engagement in the neck at touch

It seems to me that the standard chamber is pretty much 'perfect' for the shorter/lighter bullets (lots of room to play with jump without impinging upon the powder column) and perhaps a bit too short for the longer/heavier bullets, although still adequate (any jump >0.020" is starting to affect the powder column).

From my limited expertise, the long chamber seems a bit 'iffy' for the shorter/lighter bullets. If we want < 0.020" jump, then we have at most 0.170" bearing engagement in the neck. Everything I have been taught suggests that a good minimum engagement is 80% of the bullet diameter (engagement, not neck length) so about 0.230" for a 7mm. The long chamber is fine for the longer/heavier bullets.

So, if I want to be able to shoot BOTH longer/heavier bullets for long-range plinking AND 175-180 gr bullets for hunting then between the two choices I think I should go with the standard chamber, despite having a long action.

However, a BETTER answer (I think) would be a 0.250" Freebore (as 'highdrum' states).

That isn't in the cards right now, but a potential solution is to do the same thing I am about to do with my new 300 Win Mag chamber - get the 'standard' reamer (SAAMI for 300 Win Mag) and use a throating reamer and make the Freebore as long as I want. This is not ideal, I understand, since it involves two machining operations and I will never get the throating reamer as concentric with the original reamer as if I just used one reamer in the first place where all cuts were done at the same time.

Thoughts on the two issues - (i) how much bearing engagement in a neck do you really need at a minimum, and (ii) how effective is using a throating reamer to extend the Freebore?

 

[Rhovee]

.305 seems very long. Are you sure that's what Rich said? A 28 Nosler with a shorter neck throated for 195's is about .285 FB. With the neck being .050 longer you could shorter that FB a bit to cover 180 VLD-195.

 

[brittf]

.305 seems very long. Are you sure that's what Rich said? A 28 Nosler with a shorter neck throated for 195's is about .285 FB. With the neck being .050 longer you could shorter that FB a bit to cover 180 VLD-195.

Yes, actually called to verify. The 28 Sherman Magnum is about 0.260" FB (using 0.225" Lead) which is pretty spot-on for the 180-197 gr bullets, although I would agree that 0.285" might be even better for the very long bullets. It is always a tradeoff - if you want to handle the very heaviest/longest bullets and have plenty of bearing engagement in the neck, you are going to start having engagement problems as you shorten the bullet (especially the bearing length).

I should be careful about terms, since 'Freebore' and 'Lead' and 'Throat' are sometimes used interchangeably. I was taught the following use of the terms (YMMV):

• Freebore = length from end of neck to point at which the lands first appear in the chamber (includes chamfer to get from neck OD plus the Lead plus the distance it takes to get from Lead diameter to start of lands at the throat taper angle)
• Lead = length after the neck chamfer that the chamber holds a constant clearance diameter around the bullet and before the throat taper angle starts (new designs have it, older rounds such as 30-06 or the 300 Win Mag don't have Lead, only Freebore, since the throat angle starts right after the neck chamfer)
• Throat = length after Lead (if it exists) that the throat taper exists until it reaches the bore diameter

I think that Freebore (FB) is what most people are thinking of when they talk about chambers, but the variable that you are actually playing with is generally the Lead. For my SAAMI chamber 300 Win Mag, there is a Freebore of 0.147" but no Lead (by the terms above) since the Throat begins immediately after the neck chamfer. I will take a 'Throating' reamer (although by these terms, it really should be called a Lead and Throating reamer) to increase the FB to 0.250" to seat the 220 gr Lapua Scenar-L bullets.

I guess I will do something similar for the Mega. I will use the standard chamber reamer and then follow up with a 'Lead and Throating' reamer to take the 0.242" FB (based on 0.205" Lead) to 0.300" FB (by increasing the Lead to 0.263"). This is a bit more FB than the 28 Sherman Magnum (0.258" FB) but less than the 'long' Mega chamber with 0.342" FB (based on 0.305" Lead). As you can see above, the difference between the standard and long Mega chambers is 0.242" FB/0.205" Lead versus 0.342 FB/0.305" Lead. Basically, Rich increased the Lead by 0.100"

 

[Monkeygoat]

Sounds good for fine tuning those projectiles. Only thing I might want to know before setting in stone is whether any new projectiles are coming down the pipe that you might want to try. Badlands doesn't yet have a heavy 7mm... unsure if they're making one, but that would be a shame if it was too long (although I doubt it, because of the shape, a longer coal can be used, at least with the 135gr 6.5mm, without getting to the lands).

 

[brittf]

Sounds good for fine tuning those projectiles. Only thing I might want to know before setting in stone is whether any new projectiles are coming down the pipe that you might want to try. Badlands doesn't yet have a heavy 7mm... unsure if they're making one, but that would be a shame if it was too long (although I doubt it, because of the shape, a longer coal can be used, at least with the 135gr 6.5mm, without getting to the lands).

True, but at some point you just have to decide based on what you have on-hand/access to and then shoot out the barrel

The beauty of the separate 'Lead and Throating' reamer is that you are not locked in for all future chambers. If a cool new bullet comes out, then you can increase the FB in your existing barrel or wait until you shoot it out and do it on the next chamber.

The downside, of course, is that two separate machining steps imply less accuracy in the chamber, especially regarding concentricity.

 

[elkaholic]

.305 seems very long. Are you sure that's what Rich said? A 28 Nosler with a shorter neck throated for 195's is about .285 FB. With the neck being .050 longer you could shorter that FB a bit to cover 180 VLD-195.

The long action MEGA reamer is (.205) FB which would equate to about .228" in a saum because the MEGA has about .023" more neck than a saum

 

[Rhovee]

That makes more sense.

 

[brittf]

The long action MEGA reamer is (.205) FB which would equate to about .228" in a saum because the MEGA has about .023" more neck than a saum

You are the designer, so of course you know it best. I am very appreciative of your efforts to develop these cartridges and how responsive you are to people like me via phone and email.

However, in my defense, I was not asking these questions in the forum casually based on hearsay information. You and I spoke several times on the phone and I was directly inputting into a CAD model and carefully extracting the data used in the questions above. I always dislike when someone who knows little starts making statements they don't understand. That is not the case here; there must have been a simple misunderstanding somewhere along the line. My personality (as is typical for a professor) is to be very careful with my analysis and double-check before making any claims.

So, where is my CAD drawing off?

The only thing I can figure from our multiple prior conversations and your response above is that the data I have is for the long-action chamber and not the standard short-action chamber. I have gone over the material again and again but can see no indication of which chamber I have data for; oh well, no harm and no foul - except for my online reputation

So, if that is the case, then the STANDARD short-action MEGA reamer is 0.105" FB and the short-action MEGA reamer is 0.205" FB and the data I have, which I thought was for the standard reamer, is actually for the long-action reamer. Makes sense and an understandable mistake.

If so, my bad, I got the data for the long-action reamer mixed up with the data for the standard short-action chamber.

This then makes my questions above moot, since I was quite pleased with the specifications of what I thought was the standard (short-action) chamber but was, in reality, the long-action chamber.

Rich, as you know, I prefer not bothering you too often via phone, despite how gracious you are with your time. Would you mind confirming my conclusions above? In particular, the short-action MEGA reamer has a 0.105" FB and the long-action MEGA reamer has a 0.205" FB?

Thanks again, as always.

 

[brittf]

You are the designer, so of course you know it best. I am very appreciative of your efforts to develop these cartridges and how responsive you are to people like me via phone and email.

However, in my defense, I was not asking these questions in the forum casually based on hearsay information. You and I spoke several times on the phone and I was directly inputting into a CAD model and carefully extracting the data used in the questions above. I always dislike when someone who knows little starts making statements they don't understand. That is not the case here; there must have been a simple misunderstanding somewhere along the line. My personality (as is typical for a professor) is to be very careful with my analysis and double-check before making any claims.

So, where is my CAD drawing off?

The only thing I can figure from our multiple prior conversations and your response above is that the data I have is for the long-action chamber and not the standard short-action chamber. I have gone over the material again and again but can see no indication of which chamber I have data for; oh well, no harm and no foul - except for my online reputation

So, if that is the case, then the STANDARD short-action MEGA reamer is 0.105" FB and the short-action MEGA reamer is 0.205" FB and the data I have, which I thought was for the standard reamer, is actually for the long-action reamer. Makes sense and an understandable mistake.

If so, my bad, I got the data for the long-action reamer mixed up with the data for the standard short-action chamber.

This then makes my questions above moot, since I was quite pleased with the specifications of what I thought was the standard (short-action) chamber but was, in reality, the long-action chamber.

Rich, as you know, I prefer not bothering you too often via phone, despite how gracious you are with your time. Would you mind confirming my conclusions above? In particular, the short-action MEGA reamer has a 0.105" FB and the long-action MEGA reamer has a 0.205" FB?

Thanks again, as always.

One other comment that illustrates the potential confusion around chambers is the complete lack of consistency about terms.

I defined my terms in an earlier post but noted that there is no real standardization. What we are calling FB (Freebore) in the prior three posts is often called by other terms; indeed, my terminology (which I simply learned from my instructor) would call it 'Lead' and not FB. For me, Freebore is the total length from edge of neck to first engaging the lands and is LONGER than the Lead. This is (probably) consistent with what JGS uses on their drawings.

No biggie, but just cautionary...

Here is one definition:

Bullet Jump & Seating Depth: Best Practices & Conventional Wisdom

With so many events canceled and stores closed, what a great time to do some reloading! Over the pa...

precisionrifleblog.com

Here is a different definition:

RELOADERS CORNER: Bullet Jump: does it really matter? (Part One)

The distance a bullet travels to enter the lands is a topic of much concern to the precision shooter. This series takes a look at why it matters, and also when it doesn’t… Glen Zediker Bullet jump:…

mssblog.com

Even JGS, a great reamer manufacturer, can be a bit confusing, since they are not complete clear on their drawings whether the 'Lead' x-axis dimension includes the neck chamfer (usually a 45 degree angle from the neck to the Lead diameter) or just the constant Lead diameter.

https://forum.accurateshooter.com/proxy.php?image=http%3A%2F%2Fi.imgur.com%2FHCFiD88.png&hash=7d3801b4be657481ee67cd818abd8296

 

[highdrum]

@brittf a 1.5° Lead ans typical Berger hybrid nose will usually allow the bullet bearing to come ahead another 30-50k to touch lands depending on caliber from Calc freebore length based off beqring surface numbers from the berger spec sheet.

 

[brittf]

Thank you for the reply. I appreciate the information and taking the time to respond.

I tend to be overly analytical on engineering design questions. So, I generally draw everything out in excruciating detail and avoid rules-of-thumb. It probably annoys the people around me, but my students generally like the precision. At this point in my life (59 years old), I suspect I am stuck with that aspect of my personality

As an example, I am about to chamber a 28" Proof Barrel for my 300 Win Mag. Attached is my working drawing. It shows exactly how I will first use a JGS SAAMI Chamber Reamer and then follow up with a JGS Throating Reamer to get an explicit jump with a specific bullet. There are no estimates or rules-of-thumb in the drawing.

So, I really WASN'T kidding when I said that I had drawn out the 7mm Sherman MEGA chamber in complete detail. I have made similar drawings for the 7mm Sherman Max, 7mm Sherman Mega, 280 Sherman, and 28 Sherman Magnum. I evaluated each of them against a variety of bullets and looked at the resulting jump, bearing engagement length, magazine fit, and protrusion into the powder column. Any error in my prior posts was because I misunderstood the data I had access to (long-action versus short-action) and not because I was sloppy or didn't know what I was doing.

 

[brittf]

Thank you for the reply. I appreciate the information and taking the time to respond.

I tend to be overly analytical on engineering design questions. So, I generally draw everything out in excruciating detail and avoid rules-of-thumb. It probably annoys the people around me, but my students generally like the precision. At this point in my life (59 years old), I suspect I am stuck with that aspect of my personality

As an example, I am about to chamber a 28" Proof Barrel for my 300 Win Mag. Attached is my working drawing. It shows exactly how I will first use a JGS SAAMI Chamber Reamer and then follow up with a JGS Throating Reamer to get an explicit jump with a specific bullet. There are no estimates or rules-of-thumb in the drawing.

So, I really WASN'T kidding when I said that I had drawn out the 7mm Sherman MEGA chamber in complete detail. I have made similar drawings for the 7mm Sherman Max, 7mm Sherman Mega, 280 Sherman, and 28 Sherman Magnum. I evaluated each of them against a variety of bullets and looked at the resulting jump, bearing engagement length, magazine fit, and protrusion into the powder column. Any error in my prior posts was because I misunderstood the data I had access to (long-action versus short-action) and not because I was sloppy or didn't know what I was doing.

If you didn't want to download, here is a picture of the file:

 

[bamban]

Thank you for the reply. I appreciate the information and taking the time to respond.

I tend to be overly analytical on engineering design questions. So, I generally draw everything out in excruciating detail and avoid rules-of-thumb. It probably annoys the people around me, but my students generally like the precision. At this point in my life (59 years old), I suspect I am stuck with that aspect of my personality

As an example, I am about to chamber a 28" Proof Barrel for my 300 Win Mag. Attached is my working drawing. It shows exactly how I will first use a JGS SAAMI Chamber Reamer and then follow up with a JGS Throating Reamer to get an explicit jump with a specific bullet. There are no estimates or rules-of-thumb in the drawing.

So, I really WASN'T kidding when I said that I had drawn out the 7mm Sherman MEGA chamber in complete detail. I have made similar drawings for the 7mm Sherman Max, 7mm Sherman Mega, 280 Sherman, and 28 Sherman Magnum. I evaluated each of them against a variety of bullets and looked at the resulting jump, bearing engagement length, magazine fit, and protrusion into the powder column. Any error in my prior posts was because I misunderstood the data I had access to (long-action versus short-action) and not because I was sloppy or didn't know what I was doing.

Wow! Love the CAD drawing.