Three end caps give the same suppressor three different personalities. On an untuned 10.3-inch MK18, Otter Creek Labs' included partially vented cap struck the best balance.PEW Science has published its first full sound-signature evaluation of the Otter Creek Labs Infinity 556K, and the useful part is not another isolated suppressor score. The laboratory tested the same suppressor on the same 10.3-inch MK18 with three different end caps. That gives us a controlled look at what happens when a shooter trades muzzle restriction for additional gas flow.The result is mildly counterintuitive. The solid cap did not earn the highest overall rating, and opening every available vent did not either. Otter Creek Labs' included partially vented cap produced the best overall balance on PEW Science's standard untuned short-barreled AR-15.No, this is not a TFB hands-on review. The findings come from PEW Science Sound Signature Review 6.233, published July 29. PEW Science identifies itself as an independent private laboratory, but its report says this project was funded in part by Otter Creek Labs and published with OCL's permission. Readers should have that context up front. One suppressor, three gas settingsThe Infinity 556K is a 5.5-inch, .243-bore rifle suppressor intended for 5.56 NATO and 6mm cartridges. Otter Creek Labs lists a nominal weight of 14.5 ounces, a Haynes 282 body, and standard 1.375x24 HUB threads. PEW Science says the retail package is offered with or without a mount, so buyers need to check which configuration a dealer is actually selling.That one included cap matters. OCL ships the Infinity 556K with its medium-back-pressure cap, described in the PEW Science report as partially vented. A solid high-back-pressure cap and a fully vented low-back-pressure cap are separate accessories.The names make the basic trade easy to understand. Restriction generally retains more gas inside the suppressor, which can improve muzzle suppression but send more pressure back through a semi-automatic firearm. Additional venting lets the system empty faster. That can reduce the gas and ejection-port blast reaching the shooter, but the extra flow has to exit somewhere.Suppressor design refuses to be that tidy in practice. Bore size, internal geometry, ammunition, barrel length, action timing, muzzle device, and the frequency content of the resulting blast all matter. A cap with more holes is not automatically better or worse. It changes the system. How PEW Science tested itPEW Science mounted the Infinity 556K on its standard 10.3-inch MK18 test host and fired Federal XM193 55-grain 5.56 NATO ammunition. Each configuration received a six-shot string. The laboratory displayed all six signatures but used five for the analysis, maintaining its established treatment of the final shot and bolt-closing event.The tested suppressor weighed 15.3 ounces with an end cap. With a Rearden Atlas mount and single-port brake, PEW Science recorded an assembled weight of 18.9 ounces and an effective length of 6.125 inches. That is why suppressor weights pulled from different sources sometimes disagree: the manufacturer's nominal number and the weight of a complete tested configuration are not necessarily measuring the same pile of parts.PEW Science's Suppression Rating is not a simple peak-decibel figure. It is a damage-risk metric calculated from the complete measured pressure history, including frequency, duration, muzzle blast, and the signature reaching the shooter's position. Higher numbers indicate less predicted hearing risk in the laboratory's free-field test conditions.The composite results were: End capOverall Suppression RatingSolid36.4Partially vented38.0Fully vented33.8 Those numbers apply to this ammunition, host, and configuration. Moving the suppressor to a tuned 11.5-inch rifle, a 16-inch mid-length gun, or a 6mm host changes the combustion and action dynamics. The table is useful evidence, not a universal end-cap recipe. Why the middle setting workedThe partially vented cap keeps the Infinity 556K's primary bore flow restricted while opening paths from the annular space around it. According to PEW Science, that arrangement lowered the signature's frequency and reduced ejection-port blast and late gas blowback without imposing the forward-field penalty measured with the fully vented cap.Put plainly, the medium cap made better use of the suppressor's internal volume without letting everything out at once.PEW Science ranked the partially vented configuration just outside its top 10 for muzzle performance on the MK18 and inside the top 15 at the shooter's ear. The laboratory also found it to be the most balanced Otter Creek Labs configuration it has tested on the standard untuned MK18. Conveniently, it is the cap OCL puts in the box.The solid cap remains interesting. It posted a 35.9 muzzle rating and a 24.9 rating at the ear, producing a 36.4 composite result. Its restrictive setting maintained strong muzzle performance, but it did not lower ejection-port blast enough to beat the partially vented cap at the shooter's position on this untuned rifle. PEW Science notes that a solid cap paired with appropriate rifle tuning could potentially perform better. That was not tested here, so it remains a qualified possibility rather than a result.The fully vented cap went the other direction. It produced the least blowback during rapid fire but also the most severe forward blast of the three. PEW Science warns that nearby reflecting surfaces may increase the hazard penalty compared with the other configurations. A cap that feels better behind the charging handle can be less pleasant beside the muzzle. Physics does not care which end of the rifle paid for the suppressor. Dedicated bore beats the easy shortcutThe report also makes a broader technical point about bore size. The original .30-caliber Infinity can accept smaller cartridges with the appropriate end cap, but it remains over-bored for 5.56 NATO. The Infinity 556K uses a .243-inch primary bore, closer to the cartridges it is meant to suppress.In PEW Science's testing, that dedicated bore produced more consistent blast propagation across all three end caps than the larger Infinity managed on the same 5.56 host. The smaller 556K is about 1.1 inches shorter than the .30-caliber Infinity, yet its bore and vent arrangement delivered a more balanced result on the MK18.That does not make multi-caliber suppressors obsolete. A .30-caliber suppressor still earns its keep when it moves among several rifles. It does show the cost of using excess bore diameter as the easy route to lower back pressure. More clearance can move gas, but it may do so inefficiently and surrender muzzle performance. The mount and host still matterThe rear of the Infinity 556K accepts common HUB-pattern mounts, but PEW Science specifically cautions against assuming that every compatible muzzle device will preserve the measured behavior. A device that reflects too much blast into the first expansion chamber can increase effective back pressure and compromise the suppressor's early venting.That is a compatibility warning, not an invitation to start swapping parts at random. OCL should be the first stop for mount and firing-schedule guidance. Check bore alignment whenever installing a suppressor or changing its mounting system, follow the manufacturer's torque instructions, and let the unit cool before handling an end cap. Haynes 282 tolerates heat better than fingers do.Most important, suppressed supersonic 5.56 NATO still presents substantial hearing risk. The report does not declare any Infinity 556K configuration "hearing safe," and neither should we. Wear appropriate hearing protection and remember that indoor ranges, vehicles, walls, and other reflective surfaces can make the real sound field nastier than a free-field test.Leave the included cap alone unless the host gives you a reason to change it. On PEW Science's untuned MK18, the middle setting was not a compromise that finished second in everything. It won the overall comparison.