Most current 38 Super factory ammunition is loaded with .355 diameter bullets (see Table 1 on the Factory Ammunition page). However, the traditional and SAAMI specified bullet diameter for the 38 Super is .356 inches. SAAMI specification for the 38 Super barrel groove is .355 inches (bore diameter is .346 inches). The question for handloaders is which bullet diameter is best for the 38 Super, .355 or .356, or maybe something else? The traditional answer has been to use a jacketed bullet diameter that most closely matches your barrel's groove diameter but is not undersized, while lead bullets (cast, swaged) should be .001 inches larger. Plated bullets probably fall into the same class as swaged bullets because they are soft and manufactured in the same way. These rules of thumb require you to slug your barrel to see how it measures because not all barrels match SAAMI specifications (see below). Instructions for barrel slugging are given below. However, not all manufacturers agree with the idea of using bullets that match the barrel groove diameter (see below).
Barrel Groove Dimensions
Wait, don't all manufacturers make their barrels to exact SAAMI specifications? Since the SAAMI specification for a 38 Super barrel groove is .355 inches, then you should be safe just selecting .355 bullets, right? Several variables can affect barrel dimensions, such as quality control, tooling wear, and simply what the manufacturer thinks is the acceptable range of dimensions.
I slugged all the 38 Super barrels that I could get my hands on to see how they measured. I did the same to 9mm barrels and a 9X23 barrel. SAAMI specs for 9mm barrels also indicate a .355 groove diameter. As you can see in the results below, not all 38 Super barrels measured .355. Surprisingly, none of the 9mm barrels measured .355. They all slugged larger.
Started with a 0.358 inch diameter bullet. * Round count when slugged.
Not all barrel makers follow the traditional party line for recommending which bullet diameter to use for the best results. Some recommend using oversize bullets to ensures a good gas seal and positive rifling engagement. Schuemann recommends a jacketed bullet of .001 to .002 inches larger, and lead bullets .002 to .003 inches larger for the best accuracy. Nowlin also suggests slightly oversize bullets.
One concern with oversize bullets is that it raises pressure because the bullet has to be squeezed down to fit the barrel. Neither Schuemann or Nowlin seem to be concerned with this, and I know of no specific information whether this is an issue for the small amount we're talking about. If readers know of test data that addresses this, please contact me.
Undersized bullets are not necessarily bad. The fact is that many of us are probably using undersize bullets because our barrel's groove diameter is larger than .355. Accuracy can be quite good with undersize bullets. For example, I've Ransom Rest tested several loads in the CZ 75 pistols noted above with .356 barrels using .355 bullets. Some of the 5-shot groups were under 1 inch at 25 yards. Poor accuracy is not necessarily because the bullet is undersized as there are many potential causes of inaccuracy. However, I have not tested .356 bullets in these guns to see if they were more accurate or more consistent.
One other concern of undersize bullets is that gas blow-by will enhance throat and bore erosion which could reduce barrel life. But I can't say to what degree this affects the typical pistol barrel. Gas blow-by will depend on the size difference between the bullet and the barrel and how much the bullet base obturates (expands) from the force of being pushed down the bore. Bullet obturation depends on bullet hardness and pressure according to the information at the Wikipedia website.
I emailed Hornady and inquired about pistol bullet obturation. My question was: "What pressure is required for jacketed bullets to obturate? I see formulas for cast bullets, and they often indicate that it applies to cast bullets only and not jacketed bullets. Practical example: if I'm loading 9mm .355 bullets but my bore is .356, can I expect obturation at typical 9mm pressures? Or is obturation only realistic at rifle pressures?"
Here is their reply:
“Lead bullets do not in reality expand if the bullets are actually seated next to rifling such as a rifle barrel or semi auto handgun barrel. With a revolver the bullet will have a jump from the cylinder to the forcing cone of the barrel and it is at that point that the bullet will expand and the forcing cone will help to reduce the bullet down to fit the bore of the revolver barrel. For rifles or other handguns, lead bullets are generally .001" to .002" larger in diameter than jacketed bullets, this is so the lead bullet will fill out to the groove diameter of the barrel to help seal off the gases from escaping around the bullet. Jacketed bullets do not really expand as they hit the rifling of the barrel. If you have a barrel that is 9mm and the bore is .356" diameter, then you will want to shoot a .356" to .357" diameter lead bullet and a Hornady 9mm cal. 356" diameter HAP bullet.”
So, we should not depend very heavily on obturation, especially with jacketed bullets.
The Bottom Line
You can use bullets from .355 to .358 in 38 Super pistols. While it might be best to slug your bore and check its diameter, it's not necessary. If the results from my measurements of 38 Super barrels are any indication of the general trend, many barrels will not be made at the SAAMI .355 specification. In any event, even undersized bullets can provide good accuracy.
Revolver .357 and .358 bullets offer another option for the 38 Super. This is not a new idea. Hornady lists .357 bullets in their 38 Super load data along with .355 and .356 bullets. I've used Berry's 158 grain plated .358 round nose bullets in my Witness 38 Super with good accuracy. One issue with revolver bullets is whether they will feed in a semi-automatic pistol. If they have an exposed lead nose they might produce feeding problems. For example, I've tried the Remington 125 grain SJHP .357 bullet noted on the Bullets page, but it did not feed well in my guns. Note that if you use oversize bullets (.357, .358) some rounds might fit a little tight in some chambers depending on its specifications. These bullets might also feel tight if you run them through a Lee Factory Crimp reloading die.
How to slug your barrel
What you'll need:
1) Oversized, soft bullet. The bullet needs to be oversized so you'll know that it will be squeezed down as it passes through the bore. A .357 bullet will usually be big enough but a .358 bullet is ideal since few 38 Super barrels will be larger than that. Use a swaged or plated or cast bullet. Jacketed bullets are too hard and are a bitch to push through a barrel.
2) Soft tamping rod: brass or bronze is best. Wood, aluminum, nylon, etc. might or might not be sturdy enough. Do NOT use a steel rod because this can damage your bore.
4) A vice to hold the barrel. It's not absolutely necessary but it makes it much easier.
5) You'll need calipers or a micrometer accurate to at least .001 inch.
1) Remove your barrel from the gun. This isn't necessary if your slugging a revolver barrel.
2) Lube the barrel well to make the job a bit easier and to reduce the chance of bullet material tearing off and making an accurate measurement more difficult.
3) Measure the bullet before you use it so you'll be confident that it has "shrunken" from going through the bore.
4) Clamp the barrel in the vice (if you have one) with the chamber end up. Use aluminum jaws or something soft to protect the barrel finish because it might slip during pounding.
5) Drop the bullet in the barrel's chamber. Starting from this end is easier because the riflings are beveled in the chamber throat and this will help ensure that the bullet is aligned. If you're slugging a revolver barrel you have to start from the muzzle end.
6) Use the tamping rod and hammer to push (pound) the bullet though the full length of the barrel.
7) Measure the bullet diameter. The largest measurement you get is the bore groove diameter.
During various communications with barrel manufacturers, they have offered additional information, which I have included here.
Nowlin: "Barrels are made from 416 Stainless, heat treated to 40 RC hardness. 8RM bore finish. Amazing bore tolerance - near net. Barrel Groove Twist 1-14.
"Machining rifle and pistol barrels is a very complex and exacting science. The requirements for an accurate barrel are stringent. To obtain a quality barrel tolerances must be within net to .0001" for lands and groove and under 10 RMS for surface finish on the lands and groove. The barrel must be stress free from machining for accuracy and longevity. Machining or rifling processes that impart stress to the barrel, induce barrel warpage when the barrel heats up under repeated firing. The barrel and its rifling must remain optically straight after machining, rifling and repeated firing.
"Nowlin barrels are rifled using ECM (Electrode Cathode Machining). The method is stress free. A cationic mandrel tool with twist and groove depth pre-ground into the tool is inserted into the barrel which has been gun drilled and honed to a mirror finish of 8RMS. The cationic tool permits metal into the barrel to be dissipated electronically to the exact shape, with exacting tolerance, providing precise barrel groove land bore geometry that is burr and stress free.
"The process of ECM uses cations which are positive charged ions that are attracted from the barrel (anode) and flow to the cathode tools. The barrel is positively charged and the cathode tool is negatively charged. Depth of groove depends on electrical current density and time. The advantage of ECM is that it does not depend on the mechanical strength or hardness of the barrel materials.
"The ultra smooth finish produced by ECM gives less resistance on the bullet and a tighter seal between bullet and the barrel bore, which results in greater velocity and less barrel erosion (gas cutting) due to the gas leaking (venturing) around the bullet. ECM barrels are so smooth that leading from bullets and flowing from powders is mostly eliminated.
"The ECM process produces exact tolerance and dimensions because this procedure effects only the positive charged barrel (anode) and the cationic tool does not wear, which prevents varying bore dimensions."
Angela Nowlin-Reagan, Nowlin Guns, Inc.
SVI: "We suggest the Montana Gold 124gr JHP, 124gr CMJ, or 121gr IFP for 38 super major. A .356 diameter projectile can be used as well with great accuracy results. With a quality factory projectile you can expect to see a 1.5in or better group at 50 yards."
Schuemann: "As with most guns, for accuracy it is normal to have an oversized bullet. Having an oversized bullet helps maintain accuracy by properly aligning the bullet in the bore during the firing sequence..." They also recommend bullets 124 grains and up for the 38 Super. "The only portion of the bullet that will grow during the firing sequence is the bearing surface and the heavier bullets have a larger bearing surface." In their 1911 barrel manual they state: "For optimum accuracy, lead bullets should be 0.002 or 0.003 inch larger than groove diameter; copper jacketed bullets should be 0.001 or 0.002 inch larger than groove diameter."
Speer Reloading Manual #13. 1998. Ed. Allan Jones. Blount, Inc. Sporting Equipment Division. Speer, Lewiston, ID.
ANSI/SAAMI booklet Z299.3-1993. American National Standard. Voluntary Industry Performance Standards for Pressure and Velocity of Centerfire Pistol and Revolver Ammunition for the Use of Commercial Manufacturers. 1993. Sporting Arms & Ammunition Manufacturers' Institute, Inc., Wilton, Conn. USA.
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