Practical Material Penetration of M855 and M193
The Ammo we will be testing
Since we seem to be hurtling closer to a civil war every day it would be prudent for you to know what you can shoot through and what you can hide behind. So we’ll be examining various types of cover to determine how effective different types of rounds are against them. To begin we’re going to be looking at some common types of 5.56 ammunition against varying thicknesses of steel plates. We’ll be using M855 and M193 for these tests.
M855 and M193 are some of the most popular and widespread rounds in the US today. With such ready availability they would be very likely to end up your rifle and for that reason they make great candidates for testing.
The Gun
Due to the popularity of M4 style AR-15s we’ll be using a Ruger AR-556 with a 16 inch barrel to conduct the tests
Targets
Shots were made at a distance of 27 yards. All targets are mild steel and with the exception of the last plate in the ¾ inch stack have not had any significant alterations which might affect the test (the last plate in the ¾ stack has several weld beads on it). Some thicknesses were met by combining two plates to double the total plate thickness, these plates were clamped together. All targets were held in a drill press vice.
1/8 inch plate
M193
M193 zipped through the 1/8 inch plate with no problem. There seems to be a significant amount of jacket left transferred to the target which mushroomed away from the front face. Overall this created a very clean hole and as shown in the images of the backdrop generated quite a bit of secondary projectiles. This round penetrated the target so well that it did not knock the stand over, indicating a fast penetration that did not allow for much energy transfer to the target. 
Another view of the mushroom effect and of the jacket material transfer 
A view of the back of the target. This hole is relatively clean as not much material has moved. 
A view of the fragmentation. These fragments are from the bullet and the target itself and are still moving at very high speed.
M855
M855 also got through the 1/8th inch plate with no problem. Here we also see significant jacket transfer like the M193 but it appears the M855 has moved more material, indicating more energy being transferred to the target. As shown in the additional images the hole created was rough, particularly on the back face which shows a large amount of material disturbed from the exit hole 
A view of both shots. M193 at the top and M855 at the bottom. Notice a lower amount of projectiles in the backdrop 
A better view of the jacket transfer and mushrooming 
A view from the rear. Notice the amount of disturbed 
Another rear view 
¼ inch plate
M193
M193 got through the ¼ inch plate with no problem too. I was a little to the left with this shot but not far enough off to skew results. Jacket transfer is present here as well however there is notably less of it occurring and the mushrooming effect is lessened as well. Additionally, the walls of the bore itself appears to be rougher than the 1/8th inch plate and the amount of disturbed material on the rear face has increased. There also appears to be a sharp increase in spalling that appears on the back board as seen in the additional picture 
A view of the exit hole. Notice how the amount of disturbed material appears to be greater than the 1/8th inch plate 15 
A better view of the front face. Notice the decreased mushrooming effect and the rougher bore. 
A view of the spalling. A considerably larger amount has been created from this single shot than the previous two combined.
M855
M855 also made it through the ¼ inch plate with no problems. It seems to have transferred less of the jacket and created a smoother bore than M193 but it did disturb more material on the rear face. 
A view of both shots. M193 marked on the bottom. Interestingly the transferred jacket is darker than the M193 jacket, this is most likely due to greater heat somewhere in the M855’s jacket transfer process causing the metal to change color. 
Spalling is no joke. 
A better view of the bore. 
Once again a rough exit hole, disturbing a similar amount of material as it did on the 1/8th inch plate
3/8 inch plate
M193
M193 managed to penetrate the 3/8 inch plate, however it appears to have had a tougher time getting through than with the other plates. The bore is very rough and has no jacket transfer except for a very slight amount on the front face. There is very little mushrooming on the front face and a notable amount of disturbed material on the rear face. 
A view of the rear 
A view of the front face. 
A good view of the bore. 
A view of the disturbed material on the rear.
M855
M855 did not penetrate the 3/8ths inch plate. Oddly enough there is very little disturbed material on the rear, only a slight bulge (OwO). It appears most of the round’s energy acted on the front face creating a crater twice the diameter of the projectile. Most of the jacket seems to have come off the target as spalling with only a slight amount transferring to the plate. The steel core did seem to penetrate further than the rest of the lead projectile made apparent by the deeper, smaller diameter hole. 
A view of the rear face, notice the very slight bulge on the upper portion of the plate. The lack of material disturbance is very strange and warrants further investigation. It is possible that this is how the round behaves naturally, shedding the lead on impact and relying on the steel core for all of the penetration. 
A good view of the depth reached by the penetrator. 
½ inch plate
M193 – Bad Shots
I was shooting too far to the left for these shots and managed to graze the edge of the plates 3 times in a row. 
Spalling is something that needs to be carefully considered when shooting at and taking cover behind materials like these.
M193
After three failed attempts I finally managed to land a good shot. Unsurprisingly, M193 did not have enough energy to get through the ½ inch plate. It did however manage to penetrate the first ¼ inch plate in the stack and put a pretty good bulge into the second. There was very little visible jacket transfer with this shot. You’ll notice in this picture that the slug is stuck between the plates. 
A view of the front face. The slug is visible at the bottom right. 
A rear view. The bulge on the bottom left is from this shot and the ones on the right are from the missed shots. 
A view of the rear of the slug 
A view of the front of the slug. It appears the steel from the target has fused with the front face.
M855
M855 did not penetrate the ½ inch plate either. The rounds seems to have performed better on this plate than it did on the 3/8ths inch plate leaving a larger bulge that it did for the 3/8ths plate. Additionally the crater is more of how the round is expected to behave staying together better. The channel created by the steel penetrator is also not visible on this target, perhaps not even present to begin with. This will require further examination. 
½ inch plate review
Since the thickness was achieved by stacking two plates together we’ll take a look at the plates separated. M855 is on the left and M193 is on the right. The front plate is on the bottom and the rear plate is on the top. 
A view of the front face showing both shots. 
A view of the rear plate. On the left the steel penetrator is exposed. On the right is the dimple left by the slug. 
A close up of the penetrator. 
The front plate, with the hole the penetrator passed through. 
The hole on the front plate created by M193 
Both plates showing the effects of M193 
Both plates showing the effects of M855
¾ inch plate
M193
Unsurprisingly M193 wasn’t able to get through the ¾ inch plate. This shot left a pretty nice crater, but it doesn’t appear to have even made it through the first plate in the stack. On the rear nothing appears to have occurred other than the mill scale being knocked off, not even the formation of a bulge. However it should be noted that the effects on the rear plate are being altered by the weld beads on the back, while this effect is most likely negligible it should be taken into consideration. 
A view of the rear plate. The impact happens to be in the center of the bead. 
A side view showing no significant material displacement. The slug from the previous plate is visible in the background. 
A close up of the crater.
M855
As expected M855 didn’t make it through the ¾ inch plate either. It seems to have similar jacket transfer as M193 with a slightly deeper and wider crater. 
¾ inch plate review
Neither of these shots made it through the plates but both had very similar effects. 
Conclusion-
1/8th inch and ¼ inch plates offer practically zero protection from either of the bullets and create dangerous secondary projectiles and spalling. It might be better to avoid this type of cover in its entirety due to the secondary projectiles. 3/8ths inch plates may offer you some protection, especially against lower velocity rounds such as M855 but will not stop faster moving bullets such as M193. ½ inch plates and up offer consistent protection and are ideal for cover. It should also be noted that the further the bullet has to go to reach the plates the more effective they are at stopping the bullets. Examining the rounds themselves it appears that M855 projectiles do not affect the target in a consistent way, with some penetrators separating from the rest of the projectile and some sticking together the whole way through the target. From these results it seems M193 is the better round for penetrating steel barriers. Additionally the effects of spalling and fragmented bullets are devastating and must be a factor in cover selection and when selecting body armor.
I have done this test myself and found the same results: M193 is superior in virtually all uses and costs less.
Also, M193 does a vicious job on tissue while M855 tends to leave holes.
The reputation of the M16 was created using M193. I have seen pictures of M80, 308 ball, and M193 head shots. The M193 took the head off. Quite the reputation.