We consider how the government may go about proving a defendant’s prior crimes when they are an element of the charged offense.
Background
A jury convicted Breitkreutz of being a felon in possession of a firearm in violation of 18 U.S.C. § 922(g), and the court sentenced him to 15 years in prison as a career criminal pursuant to 18 U.S.C. § 922(e). When he was arrested, Breitkreutz was driving a pickup truck that had been reported stolen. Behind the seat of the truck was a rifle. He denied knowledge of the weapon when police found it, and he told the arresting officer he had borrowed the truck from one David Du-val. Breitkreutz wasn’t charged with the theft of the truck, but with possessing the rifle, a much more serious matter for someone like Breitkreutz with three prior felony convictions.
The government proceeded under a constructive possession theory: Because Breit-kreutz was driving a truck with a rifle behind the seat, it could be inferred he possessed the rifle. The owner of the truck testified he did not put the rifle in it; a woman, who was *690in the truck with Breitkreutz at the time of his arrest, testified she had no knowledge of the rifle; Breitkreutz maintained he didn’t know about it either. He said he hoped Duval could explain the rifle’s presence. But he never located Duval, so he relied mostly on his own testimony that he neither owned the gun nor knew it was there.
I
Breitkreutz contends the district court erred in denying his motion to dismiss the indictment for pre-indictment delay: The indictment was filed 31 months after he was first arrested. Preindictment delay violates due process if it “caused substantial prejudice to [defendant’s] rights to a fair trial and ... the delay was an intentional device to gain tactical advantage over the accused.” United States v. Marion, 404 U.S. 307, 324, 92 S.Ct. 455, 465, 30 L.Ed.2d 468 (1971). Breitkreutz argued (1) the government brought the federal charges to pressure him to enter a plea in an unrelated case in a different district; and (2) he was prejudiced because he could not find Duval. The district court denied the motion without an evidentiary hearing. Breitkreutz asks us to dismiss the indictment or, in the alternative, remand for an evidentiary hearing.
Given Breitkreutz’s failure to demonstrate any prejudice at all, we can’t conclude the district court erred. Breitkreutz’s argument turns entirely on the unavailability of Duval; yet, the only evidence Duval ever existed is Breitkreutz’s own testimony and the testimony of other witnesses that Breitkreutz mentioned Duval’s name to them. Equally important, Breitkreutz offered no evidence that Duval’s testimony would have been exculpatory. Because Breitkreutz has failed to discharge his burden of proving that he “suffered actual prejudice because of the delay,” his pre-indictment delay claim fails. See United States v. Horowitz, 756 F.2d 1400, 1405 (9th Cir.1985); see also United States v. Aguirre, 994 F.2d 1454, 1458 (9th Cir.1993).
II
Breitkreutz next argues the district court abused its discretion in admitting evidence of three prior felony convictions. The government offered evidence of the convictions to prove Breitkreutz had “been convicted ... of a crime punishable by imprisonment for a term exceeding one year,” an element of 18 U.S.C. § 922(g). He builds two separate arguments on Federal Rule of Evidence 403.1 First, Breitkreutz contends that the government shouldn’t have been allowed to introduce' proof of any prior felonies because he had offered to stipulate that he was a convicted felon. Second, if the government was entitled to prove up any felonies at all, it shouldn’t have been allowed to prove three when one would have fully satisfied its burden under section 922(g).
A. As the Supreme Court has noted, “the prosecution’s burden to prove every element of the crime is not relieved by a defendant’s tactical decision not to contest an essential element of the offense.” Estelle v. McGuire, — U.S.—,—, 112 S.Ct. 475, 481, 116 L.Ed.2d 385 (1991). Thus, the government is not precluded from charging and proving a prior offense by a defendant’s offer to stipulate to it: “Regardless of the [defendant’s] willingness to stipulate, the government [is] entitled to prove the [crime] by introduction of probative evidence.” United States v. Gilman, 684 F.2d 616, 622 (9th Cir.1982) (citation omitted); see also United States v. Kalama, 549 F.2d 594, 596 (9th Cir.1977).2 The unwillingness of courts to *691force the prosecutor to accept a criminal defendant’s stipulation is based on the longstanding rule that “the criminal accused cannot ‘plead out’ an element of the charged offense by offering to stipulate to that element.” Edward J. Imwinkelried, The Right to “Plead Out” Issues and Block the Admission of Prejudicial Evidence, 40 Emory L.J. 341, 357 (1991)3; see also 22 Charles Alan Wright and Kenneth W. Graham, Federal Practice and Procedure, § 5194 at 198-99 and nn. 37-38 (1978).
Indeed, we recently noted that if proof of the underlying conviction in a section 922(g)(1) action were excluded, it would “change the nature of the crime charged.” United States v. Barker, 1 F.3d 957, 960 (9th Cir. 1993). In Barker, we held the district court could not bifurcate the single offense of being a felon in possession into separate proceedings for felony status and possession because “[pjroof of the felony conviction is essential to the proof of the offense.” Id. at 959 n. 3.4
Breitkreutz nevertheless argues the district judge should have taken the stipulation into account in deciding whether to admit proof of his prior felonies under Fed.R.Evid. 403. The proffered stipulation, Breitkreutz contends, would have fully satisfied the government’s burden of proving that he was a convicted felon. Thus, any mileage the government got from the judgment of conviction above and beyond the stipulation would, of necessity, have been unduly prejudicial. See United States v. Hitt, 981 F.2d 422, 424 (9th Cir.1992).
Breitkreutz goes astray in presuming that a proffered stipulation is an alternative means of proof which the district court should consider in its 403 balancing. See Advisory Committee’s Note to Fed.R.Evid. 403. A stipulation is not proof. As we explained above, it’s a partial amendment to the defendant’s plea, a means of precluding any and all proof on a particular issue. See Sinaloa Lake Owners Ass’n v. City of Simi Valley, 882 F.2d 1398, 1400 n. 1 (9th Cir. *6921989). A stipulation thus has no place in the Rule 403 balancing process.
Breitkreutz’s suggested approach would also seriously undermine the rule that the prosecution has a right to refuse a stipulation. In every case where the defendant offers to stipulate to a prior felony — or to anything else in the prosecution’s case for that matter — the Rule 403 balance would tip against the prosecution’s evidence because it inevitably would have little if any probative value beyond that of the stipulation. Allowing stipulations to be weighed in the Rule 403 balancing process would thus defeat the rule against partial guilty pleas in most cases.
B. Breitkreutz fares better with his second argument, which challenges the introduction of three judgments of conviction to prove up a single prior felony. The district court is not required to explain precisely how it conducts its Rule 403 balancing. See, e.g., United States v. Anderson, 850 F.2d 563, 565 (9th Cir.1988); United States v. Johnson, 820 F.2d 1065, 1069 & n. 2 (9th Cir.1987). But there must be enough in the record for us to figure out whether the court understood the limits of its discretion and considered only relevant factors. United States v. Soulard, 730 F.2d 1292, 1296 (9th Cir.1984). Here, the record is unclear.
For the reasons we explain above, the government would have been entitled to introduce at least one judgment of conviction to satisfy its burden under 18 U.S.C. § 922(g). The government also invoked section 924(e), the career felony statute, which provides a statutory minimum sentence if the defendant has committed three prior felonies. The district court seems to have been misled into thinking the government was required to prove those three' felonies to the jury:
Mr. WATSON. (defense counsel): Your Honor, ... we would stipulate that the defendant has been found guilty of a prior felony exceeding one year.
Mr. GRISHAM (AUSA): Well, he’s also been charged with the career criminal statute, Your Honor, number of felonies convicted of have an effect, and that is where we named three in the indictment, and seek to introduce three here.
THE COURT: Exhibits 2, 3, 3-A and -B will be admitted.
RT 60-61.
Section 924(e), however, does not define a separate offense to be proven before the jury, but a sentence enhancement to be applied by the court. See United States v. West, 826 F.2d 909, 911-12 (9th Cir.1987); United States v. Dunn, 946 F.2d 615, 619-20 (9th Cir.1991). The jury had to find only one felony to satisfy section 922(g). Because the district court was apparently swayed by the mistaken argument that the second and third convictions were being introduced to satisfy section 924(e), we can’t be confident it properly conducted the sensitive balancing Rule 403 calls for.
It’s possible the district court let the government introduce all three priors on the theory that it is entitled to meet its burden of proof as to a single felony three different ways. But the balance between probative value and unfair prejudice shifts dramatically against the introduction of the subsequent felonies once the government proves up one. Once the government placed evidence of one of Breitkreutz’s burglary convictions into the record, proof of the other two was cumulative and therefore likely to fail the Rule 403 test. The balance might have been struck somewhat differently if the proof of each felony was shaky and there was a genuine doubt whether the government could convince the jury beyond a reasonable doubt about even one. Normally, however, prior felonies are established by documentary proof that’s hard to dispute; proof of more prior felonies adds very little of probative value and amounts to unfair piling on.
Were it clear from the record that the district court had these considerations in mind but nonetheless concluded all three convictions should be admitted, we might have deferred to its exercise of discretion. But we can’t do so on the record before us.
C. We next consider whether this error was harmless. We’ve previously noted the conflict in our circuit over the standard of review for harmless error, Hitt, 981 F.2d at 425, but its resolution is unnecessary in this case. The error here wasn’t harmless under *693either the “fair assurance” or the “more probable than not” standard. The government proceeded on a case of constructive possession. The evidence linking Breit-kreutz to the firearm was limited to the rifle’s presence behind the seat of the stolen car he was driving. No fingerprints or other indicia of ownership were introduced. To convict, the jury had to draw an inference from circumstantial evidence; this delicate judgment might well have been thrown off by its knowledge of Breitkreutz’s extensive criminal history.5
Ill
Breitkreutz also appeals his sentence. Because the district court may again have to confront his sentencing challenges, we exercise our discretion to consider two of his arguments now. See, e.g., Gregorian v. Izvestia, 871 F.2d 1515, 1526 (9th Cir.1989). Breitkreutz argues the district court should not have treated possession of a firearm by a felon as a crime of violence within the meaning of U.S.S.G. § 4B1.1. Breitkreutz is right. See United States v. Sahakian, 965 F.2d 740 (9th Cir.1992). We also agree with him that the district court should have let him attack the constitutionality of the convictions that were the basis for enhancing his sentence as a career offender. See United States v. Vea-Gonzales, 999 F.2d 1326, 1332-33 (9th Cir.1993); United States v. Clawson, 831 F.2d 909, 914-15 (9th Cir.1987).6 The district court relied on Taylor v. United States, 495 U.S. 575, 110 S.Ct. 2143, 109 L.Ed.2d 607 (1990), and United States v. Sweeten, 933 F.2d 765 (9th Cir.1991), in refusing to consider arguments about the constitutionality of the predicate offenses. These cases are inapposite because they do not deal with constitutional challenges, but with the question of whether prior offenses were violent felonies within the meaning of the statute.
REVERSED AND REMANDED
. Rule 403 states: “Although relevant, evidence may be excluded if its probative value is substantially outweighed by the danger of unfair prejudice, confusion of the issues, or misleading the jury, or by considerations of undue delay, waste of time, or needless presentation of cumulative evidence.”
. Gilman and Kalama distinguished an earlier case, United States v. Durcan, 539 F.2d 29 (9th Cir.1976), where the government refused a stipulation and introduced evidence that items imported from Canada had been obtained there by theft. In Durcan, however, the fact that the goods were stolen was not relevant to the charged offense, smuggling; that they came from Canada was all that mattered:
The evidence of the burglaries introduced in Durcan was objectionable largely because the crime of burglary was not an integral part of the offense charged, smuggling. All the prose*691cution had to prove in Durcan was where the goods were acquired, not how. In the instant case, however, the fact of a prior felony is an integral part of the offense of possession of a firearm hy a convicted felon.
Kalama, 549 F.2d at 596 (emphasis in original). Kalama stressed that evidence comprising an "integral part" of the offense need not be excluded merely because the defendant offers to stipulate. Id.
. Professor Imwinkelried does not cite this practice with particular approval. Indeed, he argues it is constitutionally suspect on equal protection grounds. Id. at 360. Breitkreutz did not raise this issue and we therefore have no occasion to consider the merits of Professor Imwinkelried's thesis.
. Our concurring brother makes much of the fact that Barker noted in passing that proof of the felony conviction could be “through stipulation or contested evidence” and that "[t]he underlying facts of the prior conviction are completely irrelevant under § 922(g)(1); the existence of the conviction itself is not.” Id. at 959 n. 3. We do not detract from these statements today: The prosecution may choose to prove up the felony by either stipulation or evidence: Barker itself recognizes that stipulations differ from contested evidence. Id. And, while the underlying facts of the felony may not be relevant, the conviction judgment or other proof — which may state the nature of the conviction — most certainly is. The concurrence reaches the opposite conclusion only by importing a Rule 403-type balancing to determine relevance under Rule 401 — an approach explicitly rejected by the advisoiy committee notes: "While situations will arise which call for the exclusion of evidence offered to prove a point conceded by the opponent, the ruling should be made on the basis of such considerations as waste of time and undue prejudice (see Rule 403), rather than under any general requirement that evidence is admissible only if directed to matters in dispute.” Advisory' Committee's Note to Fed.R.Evid. 401.
United States v. Poore, 594 F.2d 39 (4th Cir.1979), is equally inapposite. In Poore, the defendant had offered to stipulate to the earlier conviction and the government had accepted the stipulation: "[Defendant] offered to, and in fact did, stipulate that he had previously been convicted of the felony of carrying a handgun.” Id. at 40-41. Thus Poore has no bearing on whether or not the prosecutor must accept a stipulation; the issue was whether, once the parties stipulated, the district court erred by failing to redact the indictment to remove references to the nature of the offense. Poore made it clear that it was not dealing with our case by comparing its limited holding with that of United States v. Smith, 520 F.2d 544, 548 (8th Cir.1975), where "the government [was] not required to accept defendant's proffered general stipulation of conviction of felony or that of a prior felony conviction to the one relied upon in lieu of offering proof thereof.” Poore, 594 F.2d at 41.
. Fed.R.Evid. 609 doesn't change this analysis. The force of Breitkreutz's own testimony that the rifle wasn't his and he didn’t know it was under the seat was probably undercut by the jury's knowledge of his three prior convictions. This, however, doesn't help show the error was harmful because the convictions probably would have come in anyway under Rule 609 when Breit-kreutz took the stand. But neither does Rule 609 support the conclusion that introducing the prior felonies was harmless. Breitkreutz’s decision to take the stand might well have been made after the court admitted the felonies; by then, he probably figured that their introduction for impeachment would do no further damage.
. The government argues Clawson is limited to cases in which a defendant's claim regarding prior convictions can be determined without "elaborate investigations.” Appellee’s Brief at 32. We decline the invitation to create such a meaningless distinction.