CMJ vs. JHP reliability

[kgunz11]

OK, chew on this. If the MG CMJ is .010" longer when seated at the exact same COAL it will be 0.010" deeper in the case taking up more volume than the MG JHP. Fair enough?

Amen.

Those are actual measurements by the way.

[BMartens]

OK, chew on this. If the MG CMJ is .010" longer when seated at the exact same COAL it will be 0.010" deeper in the case taking up more volume than the MG JHP. Fair enough?

Amen.

Those are actual measurements by the way.

Yep, so were mine.

Whenever I get a case, or batch, of bullets I grab a handfull and measure and weigh them. At least 10 from each box and record it in my reloading log. Like to know how consistent they are and I can tell if the vary from much lot to lot.

[njl]

MG JHP 0.578"

MG CMJ 0.588"

Not touching the seating die, the JHP COAL is 1.138" and the CMJ is 1.176" COAL.

JHP has .400" out of the case.

CMJ has .430" out of the case.

Now pull out your calculators and figure the difference.

1.176 - 1.138 = 0.038

So if the bullets were of the same length, the CMJ seated to 1.176" would have 0.038" * Pi*r^2 more case volume. Since your CMJ is 0.010" longer than the JHP, and that extra length takes up space in the case, the CMJ cartridge has 0.028" * Pi*r^2 more case volume. Of course, this assumes the pieces of brass are identical and have the same internal volume before seating bullets.

Alternatively, you can work this out by subtracting the bullet length from COAL

1.138 - .578 = .560

1.176 - .588 = .588

Those results tell you how much of the OAL is space in the brass + the case head. Assuming the case heads are the same, the difference in those numbers (.588 - .560 = 0.028) gives you the length difference in the internal case volume of the cartridges.

Solving it the way you seem to be suggesting, it works out the same, but I have to ask, how did you accurately measure the amount of bullet length outside the case? Are you sure the cases were exactly equal in length. Using either of the above methods, a slight difference in case length is irrelevant. Measuring the amount of bullet outside the case introduces case length as an additional variable.

1.176 - .430 - (.588 - .430) = 0.588

1.138 - .400 - (.578 - .400) = 0.560

Here, the difference in case volume length is 0.028" in favor of the CMJ cartridge.

As mentioned, different brands (or even just different pieces) of brass can have different volumes due to differences in internal shape / construction. Other differences in bullet construction (bearing surface area, hardness, diameter) will also affect the velocity...but we'll ignore those since we seem to just be arguing about bullets and OALs.

[G-ManBart]

No time to try pictures tonight, but I measured all the various 147gr 9mm bullets I had on hand (largest selection). Zero 147gr JHPs and FMJs are essentially identical at .669-.670" long (slight variation to be expected). Precision Delta 147gr FMJ-TC's were .647" long. Montana Gold 147gr TMJ's are .635" long. Seated to the same OAL two of the four will take up the same amount of case volume. Two of the FMJs would take up less space, thus giving lower pressures and velocity. Like I said usually, not always, the FMJ will take up less space. Because there's a hole in the front of the JHP, they're normally longer to have the same weight, so they take up more case volume at the same OAL. Obviously every bullet has it's own shape and some very pointy FMJs (like the Zero 147) will be close or the same length as the corresponding JHP, but that seems to be the exception, rather than the rule.

My last chrono session I chrono'd three of these with the identical powder charge and same OAL. The MG's averaged 924fps, the PD's were 935pfs and the Zero FMJ's were 946fps. I didn't have Zero 147JHPs loaded to try (they're all in AA&A ammo so I pulled one to measure). Those were 20-shot strings and I had an SD of 4, 12 and 6 respectively. The PD's show the most variation in actual length, which is probably why the SD was higher. No gimmicks, just an accurate representation of how case volume directly leads to pressure and velocity and how typically FMJs are shorter for the same weight, and take up less case volume leading to lower velocity and pressure. R,

[BMartens]

I agree with everything you say and all of your measurements, except they are irrelevant to the OP's original post.

We are discussing Montana Gold 124 grain CMJ and 124 grain JHP bullets. Unless you have those specific bullets to measure or load, your results will be different. Overall generalities about JHP bullets fall to the side when you have specific examples and exact measurements in front of you.

The general statements about bullet length are correct, in many instances JHP bullets are longer than other profiles at approximately the same weight. Again, we are discussing specific bullets.

The samples from the lots that I received from Montana Gold measure .596" for the CMJ and .580" for the JHP. Nothing general about those measurements.

My chrono results, a couple hundred samples of each bullet, indicate that the JHP will net an average of 25 fps more velocity than the CMJ bullet in the guns I used for load development. These results held true for 2 separate guns and 4 different powders. SD's for the loads I continued developing were 4 for Silhouette and 7 for WSF for the JHP bullet. Nothing general about these results.

I dropped further development on the CMJ because I was showing flattened primers at major PF and I was able to back down .1 to .2 grains of powder with the JHP and still hit my goal of 170-174 PF with no flattened primers and no feed issues with the JHP. I shot 1000 CMJ's while developing loads so the sample size creating the results is significant.

Accuracy testing was at 25 yards, indoors, and over sandbags. Both bullets were capable of fine accuracy. I did note that the CMJ groups would start to open up just a little bit as the PF exceeded 175. The JHP produced the same basic results all the way out to 178 PF.

No overall generalities involved, just specific details of my experience with the same bullets as in the OP. I'd post some pictures but I am completely out of the 124 CMJ's.

You don't have to trust me on this, just get some of the same bullets and give it a whirl. As to why the JHP's are faster, I'm willing to just chalk it up to magic since I have no other explanation.

[G-ManBart]

I'd post some pictures but I am completely out of the 124 CMJ's.

So how are you measuring them?

The interesting thing is the OP actually asked about 125gr bullets, not the 124s you're citing measurements from.

I don't think they make a 125CMJ, but they do make a 125gr FMJ and 125gr JHP. So, you may be talking about one bullet, but either he's confused as to what he's buying, or he gave us the wrong #. He probably does mean the 124's but you have made an assumption

Edited April 23, 2010 by G-ManBart

[rvb]

[edited. sounds like the MG jhp is shorter than the cmj, so I retract my attempt at making peace.]

regardless: it all seems so simple.

given same oal, longer bullet leaves less empty case volume than a shorter bullet.

said another way, longer bullet occupies more of the case volume than short bullet.

is it that complicated?

-rvb

Edited April 23, 2010 by rvb

[BMartens]

So how are you measuring them?

The interesting thing is the OP actually asked about 125gr bullets, not the 124s you're citing measurements from.

I don't think they make a 125CMJ, but they do make a 125gr FMJ and 125gr JHP. So, you may be talking about one bullet, but either he's confused as to what he's buying, or he gave us the wrong #. He probably does mean the 124's but you have made an assumption

Dial calipers. Not using a bullet comparator, just plain old loading bench dial calipers. I believe I'm doing it like most folks would.

Yep, did make an ass-umption on the bullet weight, could be wrong but, don't think so. I mix up 124/125 all the time but, I think in this case it is relatively obvious considering the use, description and manufacturer. I will admit that 125's can throw a scare into you when they show up at the chrono stage as 124's.

[TwoShot]

Yes, I was in error when I said 125's. I am using Montana Gold 124 grain CMJ's. The comparison I was asking about was Montana Gold 124 grain JHP's, both in 9mm.

[Baer45]

I have shot all of the MG bullets without problems in my Kart barrel 1911...

[G-ManBart]

So how are you measuring them?

The interesting thing is the OP actually asked about 125gr bullets, not the 124s you're citing measurements from.

I don't think they make a 125CMJ, but they do make a 125gr FMJ and 125gr JHP. So, you may be talking about one bullet, but either he's confused as to what he's buying, or he gave us the wrong #. He probably does mean the 124's but you have made an assumption

Dial calipers. Not using a bullet comparator, just plain old loading bench dial calipers. I believe I'm doing it like most folks would.

Yep, did make an ass-umption on the bullet weight, could be wrong but, don't think so. I mix up 124/125 all the time but, I think in this case it is relatively obvious considering the use, description and manufacturer. I will admit that 125's can throw a scare into you when they show up at the chrono stage as 124's.

I meant if you're "completely out of them" to the point you can't take a photo of even one, what are you measuring?

Actually, MG's 125gr bullet would work perfectly in most 9mm barrels and really isn't all that different from the 124.

MG told me nominal spec is .590 for CMJ and .580 for JHP. R,

Edited April 24, 2010 by G-ManBart

[njl]

MG told me nominal spec is .590 for CMJ and .580 for JHP. R,

I've got cases of each of these and just measured a handful from each.

The JHP were typically .577".

The CMJ were typically .597...with one as long as .599 and one as short as .593.

I knew I'd remembered measuring them each when I first got the JHPs and noted that they were about 0.02" shorter than the CMJs...which was why I figured I should start out seating the JHP's to a COAL 0.02" shorter than I'd been doing the CMJs.

[BMartens]

So how are you measuring them?

The interesting thing is the OP actually asked about 125gr bullets, not the 124s you're citing measurements from.

I don't think they make a 125CMJ, but they do make a 125gr FMJ and 125gr JHP. So, you may be talking about one bullet, but either he's confused as to what he's buying, or he gave us the wrong #. He probably does mean the 124's but you have made an assumption

Dial calipers. Not using a bullet comparator, just plain old loading bench dial calipers. I believe I'm doing it like most folks would.

Yep, did make an ass-umption on the bullet weight, could be wrong but, don't think so. I mix up 124/125 all the time but, I think in this case it is relatively obvious considering the use, description and manufacturer. I will admit that 125's can throw a scare into you when they show up at the chrono stage as 124's.

I meant if you're "completely out of them" to the point you can't take a photo of even one, what are you measuring?

Actually, MG's 125gr bullet would work perfectly in most 9mm barrels and really isn't all that different from the 124.

MG told me nominal spec is .590 for CMJ and .580 for JHP. R,

As I mentioned in a previous response, every time I get a case or batch of bullets I weigh and measure several, at least 10, and record the data in my log. I happen to be a couple thousand rounds into load development for a new open gun so the data is pretty fresh. I do like to know how consistent my components are, it has a direct impact on the quailty of my reloaded ammunition.

The load development at this point just happens to center around these particular bullets so I have paid particular attention to their attributes and performance. I'm still working with some 115 grain MG and Zero bullets too.

I'm completely out of the CMJ's because I shot them all after I decided to stay with the JHP bullet. I've tried to be careful with 9 major ammunition as I shoot production and shoot with a lot of folks that also shoot production. Don't want somebody, including me, picking up the wrong load and shooting it in a stock production pistol. I don't even use the same bullets for 9 major and production so I can't mix them up.

It appears that my recorded measurements are pretty close to what MG tells you. My notes on the CMJ show my batch varied from .592 to .601 and varied .4 of a grain in weight. The weight varied proportionally to the variance in length. The JHP's on the other hand only varied .002 in length and .2 grains for weight.

[kgunz11]

MG told me nominal spec is .590 for CMJ and .580 for JHP. R,

I've got cases of each of these and just measured a handful from each.

The JHP were typically .577".

The CMJ were typically .597...with one as long as .599 and one as short as .593.

I knew I'd remembered measuring them each when I first got the JHPs and noted that they were about 0.02" shorter than the CMJs...which was why I figured I should start out seating the JHP's to a COAL 0.02" shorter than I'd been doing the CMJs.

Your CMJ's are much longer than mine.

MG JHP 0.578"

MG CMJ 0.588"

[MoNsTeR]

I see this thread has wandered far afield but for what it's worth, the JHPs feed absolutely 100% in my open gun.

[kgunz11]

JHP's will always feed 100%... until they don't.

[walsh]

Sorry but, simple math rules in this instance. We all measure OAL the same way, from the base of the case to the tip of the bullet, regardless of bullet profile. We measure bullet length from the base of the bullet to the tip. When measured from the same reference point, the bullet tip, the shorter bullet will leave more space in the case.

I tried to find loading data for MG JHP with no luck. It brings me to your response of a shorter bullet leaving more space.

While I know one can, and should, work-up loads, it would seem to me that with two bullets of the same weight but of different lengths, that one would take a bullet that is shorter and decrease the OAL to place it in the brass and leave the same space.

Is it that simple or is that quite wrong?

Thanks,

Walsh

[G-ManBart]

Sorry but, simple math rules in this instance. We all measure OAL the same way, from the base of the case to the tip of the bullet, regardless of bullet profile. We measure bullet length from the base of the bullet to the tip. When measured from the same reference point, the bullet tip, the shorter bullet will leave more space in the case.

I tried to find loading data for MG JHP with no luck. It brings me to your response of a shorter bullet leaving more space.

While I know one can, and should, work-up loads, it would seem to me that with two bullets of the same weight but of different lengths, that one would take a bullet that is shorter and decrease the OAL to place it in the brass and leave the same space.

Is it that simple or is that quite wrong?

Thanks,

Walsh

In many cases (not all, I'm sure) it seems that data for Hornady XTPs is pretty close to what you'll get with MG JHPs. Both tend to have a smallish cavity and similar profiles and overall lengths. I've only loaded MG's in 9/Super/SC and .40 but I wouldn't be surprised if the same held true for .45 as well.

You certainly could change the OAL to keep the amount of case volume similar, but that isn't the way most people I know go about it, for one significant reason; gun function. If you find an OAL the gun likes, it's usually better to load to that OAL and change the powder charge to adjust pressure/velocity. Some guns don't seem to care, so it's less of an issue, but I think it still makes sense to load to the OAL that seems/feels/looks to be the smoothest feeding in that particular gun.

To keep it thread related some folks load different lengths for CMJ/FMJs and JHPs because they note a difference in how the gun feeds with one or the other. Depending on bullet shape, some folks will also use different OALs to keep the nose of the bullet from catching on the ejection port when unloading. That's because it's not terribly uncommon to have the nose of the bullet hit the front of the ejection port such that it twists the case and points the primer right at the ejector. When that happens they sometimes go bang, which scares everybody and can leave brass fragments in the hands of the unwary who unload with their hand over the ejection port

Edited May 1, 2010 by G-ManBart