Armstrong stepped in for rookie Reuben Foster, who spent all of Monday playing the position. Both are vying to be the long-term replacement for Malcolm Smith, who suffered a torn pectoral Saturday and was subsequently placed on injured reserve.
"I think they're both good in the run and pass, to be honest," defensive coordinator Robert Saleh said. "Maybe I'm being biased as a coach. I think they both have great strengths. We're so detailed in our run game that I think they'll both be good in the run. We're so specific in our coverage that they'll both be good in coverage. It comes down to production and people making plays and doing things the right way. Fundamentally being sound and performing once these games start."
Armstrong acquitted himself just fine Tuesday, coming up with an interception on a Brian Hoyer pass that linebacker NaVorro Bowman deflected. Foster also had an interception after disguising a coverage and dropping into a throwing lane as Matt Barkley threw it right into his arms.
Foster and Armstrong are expected to continue sharing reps for the time being until one separates himself. Foster remains the favorite to emerge with that job, but it doesn't hurt to allow Armstrong to keep pushing him for now.
Other news, notes and observations from Tuesday's practice:
Niners coach Kyle Shanahan cut practice about 10 minutes short after undrafted rookie linebacker Donavin Newsom suffered a concussion on a collision with safety Chanceller James. Newsom was taken to Stanford Medical Center but fortunately did not have any cervical spinal injuries.
The Niners welcomed linebacker Eli Harold (concussion) and running back Tim Hightower (knee) back to practice full time and others, such as linebacker Brock Coyle (ribs) and safety Jaquiski Tartt (ribs), participated in individual drills but were held out from the team portion of the workout.
Safety Lorenzo Jerome enjoyed his longest stay with the starting defense in Tuesday's practice and didn't look out of place at all. The undrafted rookie has made a habit of delivering big hits and coming up with takeaways. He did both Tuesday. On one pass to receiver Aldrick Robinson, Jerome connected with a big hit. He also was on track to offer another big hit on a pass intended for Robinson over the middle but eased up on the unsuspecting receiver, who also pulled up and let the ball go past. Jerome also picked off a pass from Hoyer that was intended for wideout Marquise Goodwin but overthrown. Jerome appears to have a legitimate shot to win a roster spot but could bolster that case by performing well on special teams in addition to his defensive duties.
Without Joshua Garnett at left guard, the Niners seem settled on a starting offensive line of (from left to right) of Joe Staley, Zane Beadles, Daniel Kilgore, Brandon Fusco and Trent Brown. Staley had the day off Tuesday, which left first-team snaps for John Theus at left tackle and for Brown to move to the left side while Garry Gilliam got work on the right side. That's quietly a battle to watch as we go through the preseason for the primary backup tackle job.
Brown and cornerback Dontae Johnson departed Tuesday's practice with apparent injuries and neither returned before the workout was halted. Neither injury appeared too serious, though we'll know more on Wednesday.
The sloppiness of Monday's practice -- at least in terms of pre-snap penalties -- carried over a bit on Tuesday. It continues to be an issue and continues to irritate Shanahan.
The Niners' defensive line continues to get its hands on passes. Quinton Dial, Earl Mitchell and DeForest Buckner all had batted passes in Tuesday's practice while Arik Armstead continued his streak of coming up with at least one sack in every practice. Buckner also had a sack during the session.
This wasn't Hoyer's sharpest day, but there were a few good things for him to take away. He connected with running back Matt Breida on a wheel route down the sideline for a long gain that might have gone for a touchdown in a real game. Breida, by the way, looks like a natural pass-catcher who should have some plays in weekly. Hoyer also threw a picture-perfect deep ball off play action to tight end Vance McDonald for what would have been a 75-yard touchdown. Safety Eric Reid was on the wrong end of McDonald's big play.
Barkley had an up-and-down day. He started strong with a completion to Kendrick Bourne and two to Louis Murphy for sizable gains but that positive momentum vanished when he threw the aforementioned interception to Foster and safety Vinnie Sunseri broke up his next attempt.
Running backs Joe Williams and Kapri Bibbs both had impressive runs early in the practice but also each coughed up a fumble. Ball security has been pretty good for the Niners in this camp, so the turnovers were not a positive development on an overall rough day for the offense.
The 49ers are back at it on Wednesday morning before traveling to Kansas City to play the Chiefs in the preseason opener Friday night.